Hardware versions
As of this writing, there are two major variants of the PVR USB2 hardware in circulation. They can be physically distinguished by the white sticker on the underside of the device - the retail boxes are identical, unfortunately. You can also distiguish the two variants through the USB ID reported to the host when the device is plugged in.

The older device variant has a model number on that white sticker which matches the pattern "29xxx", and its reported USB ID will be 2040:2900. Throughout this documentation, the older device variant will be referred to as "29xxx hardware".
The new device variant has a model number on that white sticker which matches the pattern "24xxx", and its reported USB ID will be 2040:2400. Throughout this documentation, the newer device variant will be referred to as "24xxx hardware".
This driver is intended to work with both old and new hardware. Right now it is stable with the old hardware, and for the new hardware things seem reasonably OK as well but of course only time will tell. See the Bugs section for more details. Also, the firmware situation and chip-level modules are different in the two cases, which may impact how you set things up. The documentation on this site covers both model variants.

I've been trying to collect a census of the various model variants in circulation. You can find the current results in models.txt; please e-mail me if you would like to add your device to the tally.