Who is really suffering in Hell and Enjoying in Heaven? When a person dies, s/he goes to heaven or hell based on present life karma and any past karmas. In Buddhism, if there is no soul, who is punished in hell and who enjoys in heaven? as there is no physical body to feel pain and pleasure. If that person has learnt the art of detachment, how can s/he suffer/enjoy in hell/heaven? 
Does different realms really exists and do we travel through them?
From Milindapanha:


  -- “What is it, Nàgasena, that is reborn?”
  -- “Mind and matter (namarupa).”
  -- “Is it this very mind and matter that is reborn?”
  -- “No, it is not, but by this mind and matter deeds are done and because of those deeds another mind and matter is reborn; but that mind and matter is not thereby released from the results of its previous deeds.  


Just like an adult "inherits" results of the choices made by the child - even though most of the original body cells are gone, similarly future existence "inherits" karma from the past existence.


  --“Can there be any rebirth where there is no transmigration?”
  --“Yes there can, just as a man can light one oil-lamp
  from another but nothing moves from one lamp to the
  other; or as a pupil can learn a verse by heart from a teacher
  but the verse does not transmigrate from teacher to pupil.”


It is information and causation that is reborn, or is re-embodied. Or, as Chogyam Trungpa said, it is our neuroses that are reborn.

as there is no physical body to feel pain and pleasure?

Oh yes, there is definitely a body every time. Information always needs some kind of media, it does not exist without media... But that media could be virtual as well -- after all, form is emptiness and emptiness is form. Depending on a particular realm it may not be a crude material body. Besides, pain and pleasure do not require a body. Did you never experience mental pleasure and mental pain?

If that person has learnt the art of detachment, how can s/he suffer/enjoy in hell/heaven?

If that person has learnt the art of detachment, why would he or she be reborn in heaven or hell?

Does different realms really exists and do we travel through them?

Of course they really exist, just not necessarily in a naive way below the ground or above the sky ;) Yes, we do travel through them. Even in this world some people live in hell - don't you agree?