If you have a display manager (KDM, GDM, WDM or XDM) you should be able to do that by using the K(G/W/X)DM stop and K(G/W/X)DM start command in the directory containing your runlevels. Or if you don't have any of these installed, startx and killall X should do it for starting/stopping KDE (provided that you have the "exec startkde" line into your ~/.xinitrc file).
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