Fedora comes with GNOME as the default desktop environment, and GDM as the default login manager. The login manager is the screen you see when logging in. Like GNOME has its login manager, KDE has its own login manager KDM. While logging in you can select which desktop environment you want to use from the sessions menu of the login manager. But there seems to be no ways to switch between these login managers. But you could do this manually by adding a couple of lines to a file. Check this out.
What to do
What you need to do to switch to GDM to KDM is to open the file /etc/sysconfig/desktop, or create it if not present, and add the below two lines.
And next time you login KDM would be at work. Provided you have KDE installed in your system.
To return to GDM change the DISPLAYMANAGER enrty to DISPLAYMANAGER="GNOME".
You can do this thing also by installing the commandline application system-switch-displaymanager and by executing
yum install system-switch-displaymanager system-switch-displaymanager kdm
What’s going on inside
Now this file and these two lines might look fishy. I am describing what it actually does.
The first line DESKTOP="KDE" sets up the default desktop environment.That is, when a user selects default option from the session menu of the login manager, it would launch this desktop environment. When you select a certain desktop environment, it remembers it by writing it into the file ~/.dmrc. So When logging in later it reads the ~/.dmrc file and checks for the last user’s desktop environment configuration and the DESKTOP entry in /etc/sysconfig/desktop is ignored. If the entry in ~/.dmrc is default then the default desktop is loaded.
Next the DISPLAYMANAGER="KDE" , this selects the login manager. After the booting has completed, the script /etc/X11/prefdm file reads the /etc/sysconfig/desktop file and reads the DISPLAYMANAGER entry, sets the corresponding display manager program and executes it. So when you set it to KDE, it decodes it and kets the program to execute /usr/bin/kdm. And if the /etc/sysconfig/desktop file is not present or some display manager is provided which does not exist in the system, it simply tries GDM, KDM and other display managers, by trial and error. Because it first tries GDM, fedora come with GDM default. Check out the script /etc/X11/prefdm, and you can also change the trial and error order to first try KDM.
Also check http://fedoraproject.org/wiki/KDE.