What's The Difference Between a PVR and DVR?

That's a good question.
With the increased take-up of Digital TV, the definition seems to be getting a little blurred.

PVR is an abbreviation of Personal Video Recorder.
Hmm, right but what does that mean?
It relates to a device that can take video/audio input in real-time and record it to a hard disk drive. The recording is virtually an exact clone of the original broadcast and, unlike the old VCR's (Video Cassette Recorders), does not lose any quality in the recording process

But isn't that the same as a DVR? (Digital Video Recorder)
Well, no. The DVR was born out of the security/surveilance industry and originally related to video devices that took comparatively low quality images at time intervals slower than real-time.

PVR's are most commonly Digital TV receivers which can record to an internal or external hard disk drive. The receiver records the broadcast transport stream as a file on the hard disk.
The Healing HHR787 is a good example of a Dual Tuner High Definition PVR

However, there are some PVR's that do not contain a Digital TV Tuner and don't have an internal or external hard drive.
Such a device is the Hauppauge HD PVR. It relies on getting it's video/audio source from an external device like a HD set top box and connects to a Personal Computer, to utilise the PC's hard disk for recording.

Some PVR manufacturers term their products DVR's, which tends to muddy the waters.