To log in to the KDE or GNOME desktop or to unlock the screen, you can either enter your username and password as usual or via the virtual keyboard (xvkbd) displayed below the login field. To configure the keyboard or to access the integrated help, click the field at the left lower corner and open the xvkbd main menu.
If your input is not visible (or is not transferred to the window where you need it), redirect the focus by clicking the key in xvkbd and then clicking into the window that should get the keyboard events.
If you want to use xvkbd after login, start it from the main menu or with xvkbd from a shell.