Pumpkin fizz was a magical sweet. It was one of the sweets Fred and George Weasley brought from Honeydukes to celebrate Gryffindor's victory over Ravenclaw. It seems to be a little like a pumpkin flavoured sherbert.
Behind the scenes
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.