Basically, all types of milk of animal origin can be used. It does not matter whether you ferment cow's, sheep's or goat's milk.
Some people recommend boiling fresh or even raw milk briefly before fermentation. It does not matter which fat content you choose. However, the higher the fat content, the creamier the kefir.
Vegetable milk can also be used to make kefir. Coconut milk is particularly suitable. However, you should make sure to feed the kefir culture its usual medium - animal milk - from time to time, because with vegan alternatives you put the milk kefir on a zero diet.