Hey Aditya, PGDM (Post Graduate Diploma in Management) or PGP (Post Graduate Program) are the diploma courses and are offered by autonomous bodies like IIMs, XLRI, NITIE, MDI etc, which are not a part of any universities and run independently. While MBA (Masters in Business Administration) is a degree program offered only by universities like LPU, Amity University, Delhi University etc.
Now the difference between the two is that in India, only a University can offer degrees such as the MBA, BBA, B.Tech or B.E as per the law. So any educational institution which is not affiliated to a University cannot offer a degree. So institutes like IIMs, XLRI, NITIE, MDI etc. cannot offer MBA degree and hence offer the PGDM / PGP.