If you don't need to kill the desktop for the duration of the session you can always control+alt+function key to another virtual teletype and log in via the command line; X is running on the 5th (Ctrl+Alt+F5) one when you want to switch back to KDE.
If you really just want to get rid of X and friends, just open /etc/ttys as root and turn change the 'on' to an 'off' for the tty it runs on:
ttyv4 "/usr/local/bin/kdm -nodaemon" xterm off secure
and it will no longer start when the system starts.
Beyond that I wouldn't know, never went to far into the inerds of the X Windows System lol.