Theres an easy way to do it, which is through /etc/sysconfig/desktop. Create it if its not yet exist and add this line.
DISPLAYMANAGER=KDEValid values are:
DISPLAYMANAGER=GNOME
DISPLAYMANAGER=KDE
DISPLAYMANAGER=WDM
DISPLAYMANAGER=XDM
DISPLAYMANAGER=KDEDISPLAYMANAGER=GNOME
DISPLAYMANAGER=KDE
DISPLAYMANAGER=WDM
DISPLAYMANAGER=XDM