Is kefir fizzy?

Yes, the clue is in the name. Carbon dioxide is put in the drinks making them carbonated.

Fizzy drinks contain Carbonic acid. This acid is quite unstable and decomposes into Carbon dioxide and water. This Carbon dioxide gas comes out of the drink looking as bubbles.

Maintaining the bottle sealed.

Well if you know what sparkling is well fizzy is like the same thing as sparkling

Fizzy drinks are not fizzy in space as there is no oxygen to fizz the bubbles. The fizz in fizzy drinks is carbon dioxide coming out. In a vacuum the liquid would "boil" because of the water vapour...