Getting xscreensaver to work with KDE isn't all that complicated. Simply disable the existing KDE screensaver, and put a file in your KDE autostart to make xscreensaver kick on. Put something like this in your ~/.kde/Autostart directory:

[Desktop Entry] Exec=xscreensaver Name=XScreensaver Type=Application X-KDE-StartupNotify=false
Call it xscreensaver.desktop or something. Then run xscreensaver-demo to configure it and you're off and running. From the xscreensaver FAQ