This post focuses on KVPY.
Unfortunately I didn't give KVPY in class 11th because I wasn't aware of the deadlines.
But , as i heard from my friends - It's easy to crack and many of my friends did it!! As far as I know , the interview is a bit difficult (may be a rumor).
For class 12 KVPY aspirants , KVPY is like a cakewalk. You can crack the exam very easily if you have prepared well for JEE. Don't forget to cover the topics of JEE main Syllabus because KVPY doesn't have any "well defined" syllabus.
As far as I remember 11 of my friends appeared for KVPY and all cleared stage 1 and stage 2 both.
You will hear a lot from your friends - "Interview is very difficult , they ask tough questions and try to test your advanced skills" - wrong , wrong and WRONG !!
The interviewers are really good , humble and gentle . They assume you to be a student of class 12 and not at all "a scientist". They ask simple questions of board level. You need not look "extremely smart" or an "intelligent guy". They won't judge you by your looks. What they need is a research oriented brain.
Ofcourse most students (including me) appear for KVPY only for a certificate.
In the end - Keep calm and study!