There are more than 10 different rice varieties in Italian cuisine, but the rice used to cook Risotto have to be high in starch, round, medium or short grain and white.
Three are the most commonly used varies for risotto:
Arborio is firm and rounded, and very creamy and chewy when cooked due to its higher amylopectin starch content (is also used to make rice pudding)
Carnaroli is a very firm rice with a longer grain than Arborio and is less likely to get overcooked than other varieties.
Vialone Nano is smaller therefore cooks faster and absorbs condiments better, it is the finest and most expensive variety of rice for risotto.