What is "xvfb"

Xvfb or X virtual framebuffer is a display server implementing the X11 display server protocol. In contrast to other display servers, Xvfb performs all graphical operations in memory without showing any screen output. From the point of view of the client, it acts exactly like any other X display server, serving requests and sending events and errors as appropriate. However, no output is shown. This virtual server does not require the computer it is running on to have a screen or any input device. Only a network layer is necessary.

An alternative to Xvfb is Xdummy. While Xvfb is a standalone X server which emulates a dumb framebuffer using virtual memory, Xdummy is simply a script which uses an LD_PRELOAD hack to run a stock X server with the "dummy" video driver.