KDE is chosen from a menu on the hildon desktop. This will turn off hildon/matchbox and start KDE. Then KDE is running. KDE runs on top of the same OS so its just a matter of stopping the window manager and starting a new one while the device is running. Once in KDE, the only Hildon apps I created icons for were Application Manager and Control Panel. When running in KDE, if you Logout it wil turn black for 15 seconds and then start Matchbox/Hildon back up. So you can switch back and forth between them, though its by no means a fast process.