Nothing. KDE, Gnome and Xfce are all freedesktop (more or less) compliant: they use the same .desktop files to start applications.
KDE apps should also appear in XFCE menus unless they have a “OnlyShowIn=KDE;” line in their .desktop file (which can be commented out anyway).
The .desktop files for kde apps are in /usr/share/applications/kde4 under openSUSE.
Optionally, you can enable KDE services on startup in Xfce Settings Manager > Session and Startup > Advanced. But I don’t garantee that it works (have trouble with Gnome services).