The major differences as you go up in the CVPs will be the quality of the screen display, memory and the voice options. The 301 has a pretty tiny black and white screen, which I personally think is too small. The 303 and 305 have the same size screens, but the 305 is color and the 303 is not. The 303 will have more voice and accompaniment style options than the 301, and the 305 more than the 303, and so on. And if you're interested in recording, the 303 and 305 both have 1.5 MB of internal memory, while the 301 only has 650 KB.
Other small differences that may or may not make a difference are that the 301 does not have a microphone input or come in the mahogany finish, while all the others do. And the 305 is the only one of these three that has a video output (you can project the screen display to your own TV), and the only one of the three with GH3, which means the keyboard is more responsive, and I think more similar to a traditional acoustic piano.
I got the specs off a CVP brochure from a local dealer. It has the best side-by-side comparisons of all the CVPs, but if you can't find a brochure, the Yamaha Web site (
www.yamaha.com) lists the specs of each instrument individually.
And I agree with ranakor that if you're looking for just a piano, the CLP can offer some quality options for a lower price. But the extras on the CVP, like the score display, accompaniment, recording studio, etc. really are cool, so lucky you if you can afford one. It definitely will provide many more hours of entertainment on top of something to use for just practicing scales!
Hope this helps!