Do HDMI/DVI and DisplayPort transfer only differences or full frames?
At least at lower resolutions and in the absence of "content protection", HDMI bears a surprisingly strong resemblance to analog video used with CRTs.  A device to convert analog video to HDMI can convert voltages to pixel values and send them real time without buffering.  Likewise a device to display HDMI on a CRT (again, absent content protection) could convert the pixel values to analog values and use them to modulate the electron guns in real time as they are received.  If anyone were still making CRT-based monitors with HDMI inputs, this would allow video games using them to provide somewhat faster responsiveness than LCD panels.

Slightly confusing the issue, however, many LCD minitors will continue to show an HDMI frame (the last one received) until they get the next one, or until they time out.  It may be thus possible for a monitor to continue show a static screen while no HDMI data was being transmitted.  On the other hand, functioning devices which generate HDMI will send it out continuously, without regard for whether any of the data differs from the previous frame.  Among other things, HDMI was designed at a time when CRTs were still in use, and CRTs definitely need to be sent the image continuously, 60 or so times/second (or 30, with interlacing), in order to display it continuously and without flicker.