Switching between desktop environments without changing default
I know there is an easy command to do this, but I do not remember it and cannot seem to find it in my searches. I just want to be able to start a particular DE that I already have installed but which is not my default (for example's sake, say KDE) from runlevel 3 without changing my default one. I would like to be able to log into KDE (or another DE) and do some things whenever I feel like it by entering some command from runlevel 3, but if I enter "startx" from runlevel 3, I want to get to XFCE, as usual.
Can someone please tell me how to do this? I do not want a login manager.
Thanks in advance.