I think the different between them are the engines it has, like on the A330 A330-301 are fitted with GE, A330-321 are fitted with P&W, A330-343X are fitted with Rolls Royce (A330-342 and A330-343X have slightly different engines even though they are Rolls Royce Trent 700 but i think the performance is slightly different.
Correct me if I am wrong though, cheers
