I am running selenium through Xvfb on display number :99 like this:
/usr/bin/Xvfb :99 -ac -screen 0 1024x768x8 & export DISPLAY=\":99\" &&
It's virtual. From the man page Xvfb(1):
Xvfb is an X server that can run on machines with no display hardware and no physical input devices. It emulates a dumb framebuffer using virtual memory.
Also in the man page:
Xvfb -pixdepths 3 27 -fbdir /var/tmpThe server will listen for connections as server number 0, will have the default screen configuration (one screen, 1280x1024x8), will also support pixmap depths of 3 and 27, and will use memory mapped files in /var/tmp for the framebuffer.
xwud -in /var/tmp/Xvfb_screen0Displays screen 0 of the server started by the preceding example.