This is only about "kernel mode setting" - basically interfacing the hardware that drives your displays, sets up HDMI (if you use that) and so on. For the Raspb

This is only about "kernel mode setting" - basically interfacing the hardware that drives your displays, sets up HDMI (if you use that) and so on. For the Raspberry Pi specifically, the "fkms" for fake KMS means that you use the proprietary RPi firmware to talk to the display and setup your screen, while the kernel just throws over a framebuffer. If you use the non-fake KMS, you have the kernel do all that lower level setup but you get more direct access to the hardware that allows you to use multiple planes and so on. Why have both? The firmware, being derived from what was a set-top box, has a lot of proprietary magic to make things work with displays, also specifically with things like the official RPi display.