I think you are a little confused, KDE + Gnome are Window Managers + Desktop Environments, where a Display Manager is a different beast altogether.
A Display manager allows graphical logons -
xdm is the X display manager, It has a logo which I believe is disribution dependant (mine's the debian logo on a debian system...) a simple text box for your username and password.
gdm is the Gnome display manager, kdm is the K display manager. I don't have any experience with these, sorry.