Xvfb - a virtual framebuffer X server. 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.
Xnest -- a nested X server. Xnest is a client of the real server which manages windows and graphics requests on its behalf. Xnest is a server to its own clients. Xnest manages windows and graphics requests on their behalf.