What is the difference between Xorg and Xfbdev?

For Kernal 2.4 we will use Xfbdev and for Kernal 2.6 we will use Xorg.

I am also want to know key features in Kernal 2.6, when compare to Kernal2.4.

Thanks in advance

Xfbdev is a frame buffer only driven display server (or driver for that matter).
Xorg is a full featured display server, which includes various optimizations and advanced hardware acceleration options, and device specific drivers.

If You would be so kind to specify the device/architecture You are considering (i386, ARM, 64bit?) someone could point You to a more specific read?

Why would You want to stick to a 2.4 kernel? proprietary binary only device drivers?
Is this and embedded Linux application?

The 2.6 kernel is more advanced than the 2.4 in many regards.
Stevan C.