Hi, This is the difference:
PGDM (Post Graduate Diploma in Management) is a diploma degree issued by AICTE (All India Council for Technical Education) recognized institutions in a 2-years full-time program. B-schools offering PGDM course are autonomous bodies with industry-oriented curriculum which allows them maximum flexibility to constantly keep updating their course curriculum adapting to the industry development. It is at the par with the quality of education imparted on students pursuing an MBA degree.

PGPM (Post Graduate Programme in Management) is not a degree but a certification course. It is a typical 1-2 years program, and doesn't possess any sort of recommendation as it is generally offered by colleges without affiliations. The biggest difference is that PGDM enables you to pursue higher education like Ph.D but a PGPM program doesn't facilitate you the same.

Good Luck!