When deciding how to travel between France and Italy, take into consideration that:
all high-speed, international and night trains in France and Italy require an advance reservation. Regional trains do not require reservations, so you can just hop on and off these trains.
French TGV (high-speed) trains have a limited seat allocation. During peak periods, it's important that you book as far in advance as possible. You can reserve trains up to 3 months before your travel date.
the Thello night trains connecting Paris to Milan, Verona and Venice is not covered by the Eurail pass.