Xvfb (X Virtual Frame Buffer) is an X server that is able to run on
machines with no display hardware and no physical input devices.
Xvfb emulates a dumb framebuffer using virtual memory. Xvfb doesn't
open any devices, but behaves otherwise as an X display. Xvfb is
normally used for testing servers. Using Xvfb, the mfb or cfb code
for any depth can be exercised without using real hardware that
supports the desired depths. Xvfb has also been used to test X clients
against unusual depths and screen configurations, to do batch
processing with Xvfb as a background rendering engine, to do load
testing, to help with porting an X server to a new platform, and to
provide an unobtrusive way of running applications which really don't
need an X server but insist on having one.
If you need to test your X server or your X clients, you may want to
install Xvfb for that purpose.