What is difference between First Information Report and Police Report?

If by "first information report" you actually mean a "Preliminary Report" then that is exactly what the name implies. It is the barebones results of information known AT THE TIME (which is usually within 24 hours of the incident) and gives only the factual information known at the time it was produced, Usually the matter is still under investigation and obviously does not include any data not yet discovered.

The term "Police Report" is actually quite a flexible term and encompasses ALL of the information gathered and reduced to a narrative report. So-called "Police Reports" are viable creations and are always subject to being amended and/or added to whenever new information and/or evidence is encountered or discovered.