KMS stands for Kernel ModeSetting. That means the kernel is responsible for switching video modes and managing the video outputs (this was previously handled by Xorg).
KMS enabled hardware
Most AMD/Radeons (from R100 to R500 on Fedora 11). Most Intel hardware as well.
Switching KMS off
As root, you can use the following commands :
<enter root password>
nano -w /etc/grub.conf
to edit your current kernel's entry in
/etc/grub.conf and add nomodeset to the end of the line starting with kernel, so that it looks like :
kernel /vmlinuz-126.96.36.199-167.fc11.x86_64 ro root=/dev/rawhide/system init=/sbin/bootchartd nomodeset