Kosher in what form of Judaism? Orthodox Judaism grew out of the religion of the Pharisees, which kind of took over the religion some time after the temple was destroyed (which was the power base of the Saducee/Zadokite priests who were powerful in the religion earlier.) The main branches of Orthodox Judaism are Sephardic and Ashkenazic branches of Judaism. Both Orthodox groups would not allow the eating of milk and meat together because of traditional interpretations. The Old Testament actually forbids eating a calf or goat cooked in its mother's milk, but Orthodox Jews do not allow chicken and cheese sandwiches. No milk and meat.
So if there is a kosher cheese pizza from a kosher kitchen, it would be considered kosher. Otherwise, if you put kosher sausage on it along with the cheese, Orthodox Jews would not consider it to be kosher.
If I understand correctly, cheese could be nonkosher if the rennet used to make the cheese was from a calf that was not slaughtered kosher.
There are Kariate Jews, which only believe the Old Testament, but not the Talmud and some of the other traditional writings. I am not sure, but I suspect they would be okay with kosher sausage if the animal it came from was slaughtered kosher and the cheese was kosher and not from the mother of the animal used to make the sausage, they may be okay with pizza.
You couldn't have pork toppings, of course. Pork is not kosher.
Btw, I read that some giraffe was kosher if slaughtered properly.