Pumpkin fizz is a magical sweet. It is one of the sweets Fred and George Weasley bring from Honeydukes to celebrate Gryffindor's victory over Ravenclaw. It seems to be a little like a pumpkin flavoured sherbert (See Lemon Sherbert).
Pumpkin Fizz is also a beverage served at The Wizarding World of Harry Potter. The beverage is similar to the pumpkin juice sold there, but carbonated.