Are There Levels of Jannah?
Question

People in Paradise will be of varying status; some of them will be of high status, and some of them will be lower in status, and so on. Will the one who is in a lower status be able to attain the same as the one who is above him will attain?
Praise be to Allah.Paradise is of different levels 
One of the things that Muslims believe is that Paradise is of different levels, and that Allah has promised those who obey Him places to dwell in Paradise in return for their acts of obedience. The difference among the people of Paradise in status and levels is only due to the difference among them in terms of acts of obedience and righteousness in this world.
Allah, may He be exalted, says (interpretation of the meaning):
“Look how We have favored [in provision] some of them over others. But the Hereafter is greater in degrees [of difference] and greater in distinction” [al-Isra 17:21]
“And for all are degrees from what they have done. And your Lord is not unaware of what they do” [al-An’am 6:132].
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
“Paradise is of different levels, that vary greatly, and the pious, believing friends of Allah will be in those different levels according to the level of their faith and piety.” (Majmu‘ al-Fatawa, 11/188. See also: Tafsir as-Sa‘di, p. 274)
In the answer to question no. 27075 , we have discussed the deeds by means of which the Muslim may attain high levels in Paradise.
People of Paradise will vary in the bliss they enjoy in Paradise 
One of the matters concerning which there can be no doubt is the fact that the people of Paradise will vary in the bliss they enjoy in Paradise, according to the levels they attain. The one who dwells in al-Firdaws (the highest level of Paradise) will not be like one who dwells in lower levels of Paradise.
Allah, may He be exalted, has described the gardens of Paradise that will be granted to the one who fears the standing before Him, may He be glorified and exalted. He says (interpretation of the meaning):
“But for he who has feared the standing before his Lord are two gardens –
So which of the favors of your Lord would you deny? –Having [spreading] branches…” [ar-Rahman 55:46-48].
Then He describes them, then He says:
“And below them both [in excellence] are two [other] gardens” [ar-Rahman 55:62].
Thus it is clear that the gardens of Paradise differ from one another according to the good deeds of their occupants and their status before their Lord.
Abu Sa‘id al-Khudri (may Allah be pleased with him) narrated that the Prophet (blessings and peace of Allah be upon him) said: “The people of Paradise will see the people of the highest places in Paradise above them as you see a brilliant star far away on the horizon in the east or the west, because of the difference in status between them.” They said: O Messenger of Allah, is that the status of the Prophets that no one else will attain? He said: “No, by the One in Whose Hand is my soul. They are men who believed in Allah and believed in the messengers.” (Narrated by al-Bukhari, 3083) and Muslim, 2831)
According to another report, “The people of the highest degrees of Paradise will be seen by those beneath them as a star is seen on the horizon. Abu Bakr and ‘Umar will be among them, and how blessed they are!” (Narrated and classed as hasan by at-Tirmidhi (3658). Also narrated by Ibn Majah (96), from the hadith of Abu Sa‘id. Classed as sahih by al-Albani in Sahih at-Tirmidhi.)
Al-Qurtubi (may Allah have mercy on him) said:
“It should be understood that these high places will vary in elevation and description, in accordance with the differences between their occupants in terms of righteous deeds. So some of them will be higher than others, and more elevated.
With regard to the phrase “a brilliant star far away on the horizon in the east or the west”, what is meant is that the star, when it is rising or setting, is far from people’s vision, so it appears small because it is so far away.” (At-Tadhkirah fi Ahwal al-Mawta wa’d-Dar al-Akhirah, p. 398)
Can those in the lower levels of Paradise attain what is in the highest levels? 
What has been mentioned above clearly indicates that it is not possible for those who are in the lower levels of paradise to attain what is in the highest levels, because they did not do that which makes them deserving of attaining those levels.
If all the people of Paradise were to share in the bliss that Allah has prepared for those who are above them, then there would be no wisdom in the variation of status and degree! By Allah’s perfect justice, those who are deserving of Paradise will not all be the same in degree or level of bliss. Variation between people in this world in terms of faith and obedience leads to variation in their status and standing before Him, may He be glorified and exalted.
Al-Qurtubi (may Allah have mercy on him) said:
“With regard to the words “By the One in Whose Hand is my soul, there are men who believed in Allah and believed the messengers”, he did not mention any righteous deeds or anything else, except faith and belief in the messengers. That is so that it would be understood that he was referring to complete faith and belief in the messengers without asking for a sign and without hesitation. Otherwise, how could these high levels be attained by merely believing and accepting the messengers, which ordinary Muslims do. If that were the case, then all those who affirm the Oneness of Allah would be in the highest levels and degrees of Paradise, and this is impossible.” (At-Tadhkirah, p. 398)
Will there be envy or resentment in Paradise?
There will be no envy or resentment in Paradise. Concerning that, Allah, may He be exalted, says (interpretation of the meaning):
“And We will remove whatever is in their hearts of resentment, [so they will be] brothers, on thrones facing each other” [al-Hijr 15:47].
Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The first group to enter Paradise will look like the moon when it is full. They will not spit or blow their noses or defecate therein. Their vessels there will be of gold, and their combs will be of gold and silver, their incense burners will be of aloeswood and their sweat will be musk. Each of them will have two wives, the marrow of whose calves will be visible from beneath the flesh because of their beauty. There will be no dissent or enmity among them and their hearts will be as one, and they will glorify Allah morning and evening.” (Narrated by al-Bukhari, 3073) and Muslim, 2834)
Think about this hadith:
Ibn Mas‘ud (may Allah be pleased with him) narrated that the Messenger of Allah (blessings and peace of Allah be upon him) said: “ The last one to enter Paradise will be a man, who will walk a little, then fall on his face, then the Fire will touch his face and leave its mark on it, then when he has passed it, he will turn back to look at it and will say: Blessed be the One Who has saved me from you; Allah has given me something that He has not given to any of the first and the last.” (Narrated by Muslim, 274)
This is the state of the last of the people of Hell to be brought forth from the Fire, and the last of the people of Paradise to be admitted to Paradise; he will think that he has attained something that no one else, of the first and the last, has attained, and he has not even entered Paradise yet, so how about when he does enter it?!
Ibn ‘Atiyyah al-Andalusi (may Allah have mercy on him) said:
“Everyone in it will be granted contentment with his situation, and he will not believe that anyone has been given something better than him. We know from the religious texts that the people of Paradise will vary in status, commensurate with their good deeds, and commensurate with the grace of Allah to whomever He wills.” (Al-Muharrar al-Wajiz fi Tafsir al-Kitab al-‘Aziz. 2/91) See also: al-Jannah wa’n-Nar (available in English under the title Paradise and Hell) by Shaykh ‘Umar Sulayman al-Ashqar (154-163).
The means of being raised in status in Jannah
There, by the grace of Allah, may He be exalted, to His believing slaves, He will not deprive any person of his status in Paradise and put him in a lower level (than he deserves), but He may bestow His grace upon whomever He wills of His slaves, and raise his status in Paradise. The means of being raised in status in this manner include the following:

The intercession of the intercessors

There are two hadiths which speak of this:   

Abu Musa (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) … raised his hands and said: “O Allah, forgive ‘Ubayd Abu ‘Amir,” until I could see the whiteness of his armpits. Then he said: “O Allah, on the Day of Resurrection make him above many of Your creation.” (Narrated by al-Bukhari, 4067) and Muslim, 2498)

Abu ‘Amir is al-Ash‘ari; he is the paternal uncle of Abu Musa (may Allah be pleased with him).

Umm Salamah said: The Messenger of Allah (blessings and peace of Allah be upon him) entered upon Abu Salamah and his eyes were fixed open. He closed them, then he said: “When the soul is taken, the gaze follows it.” Some of his family wailed and he said: “Do not pray against yourselves except for good things, for the angels say amin to whatever you say.” Then he said: “O Allah, forgive Abu Salamah and raise him in status among those who are guided, and take care of his family who are left behind. Forgive us and him, O Lord of the Worlds, and make his grave spacious for him, and illuminate it for him.” (Narrated by Muslim, 920)

Ibn al-Qayyim (may Allah have mercy on him) said:
The second type (of intercession): The intercession of the Prophet (blessings and peace of Allah be upon him) for increased reward and higher status for some of the believers.” (Hashiyat Ibn al-Qayyim ‘ala Mukhtasar Sunan Abi Dawud, 13/56)
Shaykh Muhammad ibn Salih al-‘Uthaymin (may Allah have mercy on him) said, explaining the types of intercession:
The third type: intercession for high status for the believers. This may be understood from the du‘a (supplication) of the believers for one another, as the Prophet (blessings and peace of Allah be upon him) said when he prayed for Abu Salamah…
Majmu‘ Fatawa ash-Shaykh Ibn ‘Uthaymin (9/326).

A child’s supplication and prayer for forgiveness for his parent

Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “A man will be raised in status in Paradise and will say, Where did this come from? And it will be said: From your son’s praying for forgiveness for you.” (Narrated by Ibn Majah, 3660); classed as hasan by al-Albani in as-Silsilah as-Sahihah, 1598)

Parents being raised to the status of their children and vice versa.

Allah, may He be exalted, says (interpretation of the meaning):
“And those who believed and whose descendants followed them in faith - We will join with them their descendants, and We will not deprive them of anything of their deeds. Every person, for what he earned, is retained” [at-Tur 52:21].
Ibn 'Abbas said: Verily Allah will raise the offspring of the believer to his level (in Paradise), even if they were lower than him in terms of righteous deeds, so that he may delight in them being with him. This is a sahih report and may be deemed marfu‘. (See: as-Silsilat as-Sahihah, 2490)
Shaykh Muhammad ibn Salih al-‘Uthaymin (may Allah have mercy on him) said:
“Allah, may He be exalted, says (interpretation of the meaning): “And those who believed and whose offspring followed them in faith - We will join with them their descendants” [at-Tur 52:21]. The offspring whose faith follows that of their parents are small children (minors). So Allah is saying, “We will join with them their offspring” i.e., We will cause their offspring to join them in their levels (in Paradise).
As for adult children who have gotten married, they will be in their own levels in Paradise, and will not join their parents, because they have offspring of their own, so they will be in their own abodes. As for small children who followed their parents, they will be lifted up to be with their parents. This lifting up does not imply any detraction from the reward and status of the parents, hence Allah says: “and We will not deprive them of anything of their deeds” [at-Tur 52:21]. The word translated here as deprived means detract or take away. In other words, their offspring will join them, but He did not say that the parents’ level will be demoted commensurate with the degree by which the offspring are raised in status; rather He says, “and We will not deprive them of anything of their deeds.” (Tafsir al-Quran min al-Hujurat ila al-Hadid, p. 187)
Will people visit each other in Jannah?
There are some hadiths which indicate that when the people in the higher levels want to visit those who are lower than them, they will go down to them, but the people of the lower levels will not ascend to the higher levels. But these reports are not sahih. Examples of that include:

It was narrated that Abu Salam al-Aswad said: I heard Abu Umamah say: A man asked the Messenger of Allah (blessings and peace of Allah be upon him): Will the people of Paradise visit one another? He said: “Yes, the people of the higher level will come down to the people of the lower level, and they will welcome them and greet them with salam, but the people of the lower level will not be able to ascend to the higher levels, because their good deeds fell short of that.” (Narrated by Ibn Abi Hatim in his Tafsir, 10/3371)

There is some weakness in it, because its isnad includes Sa‘id ibn Yusuf.
Yahya ibn Ma‘in said: He is da‘if in hadith; he is not strong. (Ad-Du‘afa wa’l-Matrikin by Ibn al-Jawzi, 1/327)

Abu Umamah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The people of Paradise will visit one another on saddled camels. The people of the higher levels will visit those who are lower than them, but those who are lower than them will not visit those who are in the higher levels, except two who loved one another for the sake of Allah; they will visit one another in Paradise wherever they wish.” (Narrated by at-Tabarani in al-Mu‘jam al-Kabir, 8/240)

Its isnad includes Bishr ibn Numayr, who is matruk (rejected); he was accused of fabricating reports.
Al-Haythami (may Allah have mercy on him) said: Its isnad includes Bishr ibn Numayr, who is matruk (rejected). (Majma‘ az-Zawaid, 10/496)

A similar, marfu‘, report was narrated by Abu Na‘im al-Asbahani in Sifat al-Jannah (no. 421) via Ja‘far ibn az-Zubayr and Bishr ibn Numayr, from al-Qasim, from Abu Umamah.

But Ja‘far ibn az-Zubayr is matruk (rejected), and was accused of fabricating reports.
Bishr ibn Numayr is also matruk, as noted above.

“When the people of Paradise enter Paradise, and brethren miss one another, the couch of one will travel to the couch of another, and so on, until they are all gathered together, then they will recline, and one of them will say to the other: Do you know when Allah forgave us? And his companion will say: Yes, the day when we were in such and such a place, and we prayed to Allah, and He forgave us.”

This is a da‘if (weak) hadith. See: as-Silsilah ad-Da‘ifah (5029).
The most that can be understood from the words of the scholars concerning this issue is what was narrated by Abu Na‘im al-Asbahani in Sifat al-Jannah (422) from Humayd ibn Hilal, who said: We heard that among the people of Paradise, the one who is in a higher level will visit the one who is in a lower level, and the one who is in the lower level will not visit the one who is in the higher level.
Humayd was one of the Tabi‘in; the isnad going back to him is sahih (sound). Allah knows best what the truth is concerning this matter.
And Allah knows best.