The three main risotto rices are Arborio, Carnaroli and Vialone Nano. Carnaroli has a high starch content and makes the creamiest risotto; it is often preferred by chefs as the starchy centre stays chewily firm in spite of absorbing plenty of liquid.
Arborio makes a humbler, lighter risotto, while Vialone Nano, which has shorter, rounder grains, is reputed to have the highest absorption ability. This makes it suitable for wet, soupy risottos and the traditional choice for risi e bisi (rice and peas).
Taken from The Daily Telegraph