PiScreen isnt supported on XBMC. However there is a work around to copy what is display on the HDMI port(fb0) to the PiScreen.
To do this, we will need Framebuffer Copy.
pi@raspberrypi ~ $ sudo apt-get install cmake
pi@raspberrypi ~ $ git clone https://github.com/tasanakorn/rpi-fbcp
pi@raspberrypi ~ $ cd rpi-fbcp/
pi@raspberrypi ~ $ mkdir build
pi@raspberrypi ~ $ cmake ..
pi@raspberrypi ~ $ make
pi@raspberrypi ~ $ sudo install fbcp /usr/local/bin/fbcp
pi@raspberrypi ~ $ git clone https://github.com/tasanakorn/rpi-fbcp
pi@raspberrypi ~ $ cd rpi-fbcp/
pi@raspberrypi ~ $ mkdir build
pi@raspberrypi ~ $ cmake ..
pi@raspberrypi ~ $ make
pi@raspberrypi ~ $ sudo install fbcp /usr/local/bin/fbcp
To use Framebuffer Copy the console needs to be on fb0. If it isnt already there, you can change it to fb0 using this command;
pi@raspberrypi ~ $ con2fbmap 1 0
The run framebuffer copy in the background
pi@raspberrypi ~ $ fbcp &
You should now have fb0 mirrored onto fb1(PiScreen).