Hi

There are three categories of kvpy exam-

Stream SA

Stream SX

Stream SB

Stream SA: Students enrolled in XI Standard (Science Subjects) during the given academic year

Stream SX: Students enrolled in XII Standard/ (+2) (Science subjects) during the given academic year

Stream SB: Students enrolled in the 1st year of undergraduate program in Basic Sciences

So a student of class 12 will lie in the category SX of kvpy exam.

Hopd this helps.