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Kubuntu uses SDDM by default. I've tried hard to make it display a virtual keyboard, but to no avail. The solution I ended up using is switching to LightDM.
Open your Konsole, type this in:
sudo apt-get install lightdm
Some time later a menu will be displayed, click 'ok', then choose 'lightdm'.
Now go to the Software Center and type 'lightdm'. Install 'LightDM GTK+ Greeter Settings'. Using this app you can configure your new login screen to your liking, including picking an on-screen keyboard.
Now just reboot and you're done.