I could easily be wrong but I believe...
The "P" in ATMEGA644P and ATMEGA644PV indicates this...
The "V" in ATMEGA644PV and ATMEGA644V indicates a "low voltage" (or extended voltage) processor.
The "10" indicates a maximum clock speed of 10MHz.
The "20" indicates a maximum clock speed of 20MHz.
I have no idea what the "PU" signifies.