Wikipedia
Xephyr
Xephyr is a display server implementing the X11 display server protocol based on KDrive which targets a window on a host X Server as its framebuffer. It is written by Matthew Allum. Xephyr is an X-on-X implementation and runs on X.Org Server and supports Glamor. Future versions could make use of libinput. Replacing Xephyr with the xf86-video-dummy and xf86-video-nested drivers in the normal X.Org server is being considered as part of X11R7.8.