I believe two different types of rice are used for Paella and Risotto. The risotto rice has a much creamier and sticky texture to it when cooked (it has a higher starch content). As for the rice used in Paella it sticks together but does not have the same creamy aspect to it.
Risotto Rice - Arborio, Baldo, Carnaroli, Maratelli, Padano, Roma, and Vialone Nano.
Paella Rice - Bahia, Balilla, Bomba, Senia, and Calasparra.
I tend to use the Arborio for my risotto and just general white rice for paella but its just down to taste I guess.