Xv is The XFree86 X Video Extension (Xv) Device Dependent Layer, an extension to XFree86 that allows applications access to hardware accelerated video overlaying and scaling. Without it, some TV viewing applications will cause flicker by making your desktop redraw whenever their window is resized or partially obscured or revealed by another window.
Xv was introduced in XFree86 4.0 (see the Xv(3x) man page). To find out if you're using it, try xvinfo(1). xvattr is also useful to set parameters for it.
If you have TvTunerCards and you want them to use Xv, you need to load a driver in your XServer. Add
to the Module section of its configuration (for XFree86: /etc/X11/XF86Config; for X.org: /etc/X11/xorg.conf).
See also Video4Linux