What are all the bhawana (meditation) methods in Buddhism? There are two types of meditations described as "Samatha" and "Vipassana"
I have heard that there are several types of samatha bhawana (40 kamatahan). Can you please elaborate this based on Theravada Buddhism
As answered here, the forty samatha meditation objects are:

1-10: The Kasinas

These all lead to the four jhānas; they are practised by creating a disk and focusing on it, while repeating, e.g., "earth, earth..."

1. Earth
2. Water
3. Fire
4. Air
5. Blue
6. Yellow
7. Red
8. White
9. Light
10. Space

11-20: Foulness

These lead to the first jhāna; they are practised by contemplating a corpse in various states of decay

11. Bloated
12. Livid
13. Festering
14. Cut Up
15. The Gnawed
16. Scattered
17. Hacked and Scattered
18. Bleeding
19. Worm-infested
20. a Skeleton 

21-30: Recollections

Body and Breathing bring jhāna (body only brings the first jhāna, breath brings all four); the other eight bring only access concentration; they are cultivated by reflecting on the object, e.g. "Buddha, Buddha"

21. Buddha
22. Dhamma
23. Sangha
24. Morality
25. Generosity
26. Angels
27. Death
28. Body
29. Breathing
30. Peace

31-34: The Divine Abidings

The first three bring about the first three jhānas; the fourth brings about the fourth jhāna; they are cultivated by taking beings as an object, e.g. "May all beings be happy"

31. Love 
32. Compassion
33. Joy 
34. Equanimity 

35-38: The Immaterial States

These bring about the fourth jhāna; they are cultivated by using the fourth jhāna as a base, then expanding and refining the object

35. Limitless Space
36. Limitless Consciousness
37. Nothingness
38. Neither Perception Nor Non-perception

39. Perception of Repulsiveness in Nutriment

This brings about only access concentration; it is practiced by reflecting on food as one collects and eats it.

40. Defining of The Elements

This brings about only access concentration; it is practiced by breaking the body up into the four elements.

All forty meditation subjects are described and taught in detail in the Visuddhimagga; highly recommended for those interested.