KVPY SX eligibility criteria:

Students enrolled in XII standard / (+2) during the academic year 2017-18 and aspiring to join an undergraduate program in Basic Sciences (B.Sc./B.S./B.Stat./B.Math./Int. M.Sc./M.S.) for the session 2018-19 provided they have secured a minimum of 75% (65% for SC/ST/PWD) marks in aggregate in MATHEMATICS and SCIENCE subjects (Physics/Chemistry/Biology) in the X Standard Board Examination.
They must secure 60% (50% for SC/ST/PWD) marks in aggregate in MATHEMATICS and SCIENCE subjects (Physics/Chemistry/Biology) in the XII standard Board Examination before taking up the fellowship, if awarded.

For SX stream, there are total 120 questions out of which 80 questions of part 1 carries one mark each and part 2 carrying 40 questions of 2 marks each
Negative marking scheme states, deduction of 0.25% marks for wrong attempts in part 1 and 0.50%  marks will be deducted for wrong attempts in part 2.

Rest information on KVPY you can check below:
All about KVPY

Good luck.