Linux: is it possible to see only kernel space threads/process?

Kernel processes (or "kernel threads") are children of PID 2 (kthreadd), so this might be more accurate:

ps --ppid 2 -p 2 -o uname,pid,ppid,cmd,cls

Add --deselect to invert the selection and see only user-space processes.

(This question was pretty much an exact inverse of this one.)

In 2.4.* and older kernels, this PID 2 convention did not exist yet.

Kernel threads do not use RAM at all (or at least are displayed not to use any):

ps -eo cmd,vsize,rss | grep -E ' 0 +0$'