Xvfb (X Virtual Frame Buffer) is an X Windows System server
that is capable of running 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.