The Kishore Vaigyanik Protsahan Yojana (KVPY) is an on-going National Program of Fellowship in Basic Sciences, initiated and funded by the Department of Science and Technology, Government of India, to attract exceptionally highly motivated students for pursuing basic science courses and research career in science.

As you have mentioned  that you are a class 12 th student , so this means while filling up the KVPY form you would have opted the SX stream , You may note that unlike Class 11 KVPY SA , the KVPY SX stream gives you option to choose one subject from Maths and Biology and this option is being provided to you at the time of exam only, This option is also provided in both the sections separately also . in case you want to attempt maths in one and bio in the second section

You must also take a note that you are free to attempt both maths and biology in the paper, in such a case best out of the two would be considered

Now as you are mentioning that you are a PCM stream student , so I am assuming you don’t have knowledge of +1 +2 Biology so it would be best if you wholeheartedly prepare maths and attempt that only in the paper

Hope your doubts got resolved

Regards