Hello Manish!
I'm sorry to inform you that you can't give the Ph.D entrance exam itself in Hindi medium, let alone study the complete doctorate in Hindi. All Ph.D entrance exam is conducted in English language, hence the education that follows is also continued in English.

If you really want to pursue Ph.D in any subject, you'll need to score a minimum of 50% marks in aggregate of your Post Graduation examination and have your course done from a well-recognised University before the age of 28.

Good luck for your future!