My local supermarket stocks arborio and carnaroli rices for making risotto, and also stocks "paella rice". I have, in the past, used them interchangeably in both risottos and paellas. Wikipedia handily summarises the difference between carnaroli and arborio here, but I can't find a description of the differences between these and a paella rice (aside from country of origin).
Note: I'm aware that the rice is cooked differently in a paella compared to a risotto -- I'm asking about the differences in the rice itself, not the cooking method.