The P2000 is basically an updated compact USP, by updated i mean it does away with the horrendous USP grip (my opinion) for slimmer, more comfortable grip with interchangable backstrap and doesn't have the stipling like the USP.
It also has a standard acc. rail not the HK proprietary rail, which is a major bonus!
There is no manual safety/decocker like the USP
The P30 takes over where the P2000 left off, except it has interchangable side panels on the grip, to adjust palm swell also the grip texture is like that of the Sig pistols.
There are some other intenal tweeks HK did but it would take too much time to go over here, but they're both basically upgrades based on the USP.