What are the differences between the 'VESA' driver and the 'fbdev' driver for Xorg? Both have no hardware acceleration a.f.a.i.k.
The fbdev driver supports all hardware where a framebuffer driver is available.
The vesa driver supports most VESA-compatible video cards. There are some known exceptions, and those should be listed here.
They are written for different kinds of devices. Frame buffer devices do not have to be VESA-compatible (although an abstraction driver from VESA to framebuffer called vesafb exists), or physically present, for that matter.