What is the real difference between very high and xtreme?
I think the game is darker and less clear on xtreme. I can play it smoothly with good fps on xtreme and the same
on veryhigh. Should i download dx11 patch to get the xtreme to look more attractive as veryhigh? or is xtreme for 3d glasses and 3d TV?

Extreme is just a higher level of detail, more particles, tessellation and POM. Very high is more suited to midrange PCs.

i played with no DX 11 after high-res textures patch
i was not noticed with any difference really
Crysis 1 still kicks an ass