Dear Shwetha,
First of all, professionally there is no difference between post-graduation in Business Management (PGBM), Post-Graduation Diploma in Business Management (PGDBM), and an MBA degree. Technically, an institute affiliated with a university gives an MBA degree like FMS, JBIMS, and autonomous institutes give a diploma in management (PGBM, PGDBM) like IIM's, IIFT, MDI, XLRI, etc.
It is not true that you can't do a PhD after PGDBM because all the IIM's, IIFT, MDI, XLRI, and many other institutes offer FPM which is equivalent to a PhD. and again they give FPM because they can't give PhD degrees as they are autonomous institutes and not affiliated with universities. People usually say there is no big difference between an MBA and PGDBA or PGDBM. It is a marketing tip to sell the seats available in their institutions. But, there is a big gap between both of them and one will know it when he comes out of his college with his valuable and costly certificate. No one in the market will agree that a PGDBA or PGDBM is equivalent to an MBA. And with that point, they try to depreciate you or degrade you.
Comparing PGDM and MBA is akin to comparing a Polytechnic Computer Science Diploma to a B.E. Computer Science diploma. PGDM is more functional or more towards how to handle day-to-day management duties at a lower/middle-level management; MBA is more at an executive or senior level. PGDM is tactical, and MBA is strategic.
In the US, they call PGDM as PMP.