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 MBA degree. Technically an institute affiliated with an university gives an MBA degree like FMS, JBIMS and autonomous institutes give diploma in management [PGBM, PGDBM] like IIM’s, IIFT, MDI, XLRI, etc.
It is not true u can’t do PhD after PGDBM because all the IIM’s, IIFT, MDI, XLRI and many other institutes offer FPM which is equivalent to PhD. and again they give FPM because they can’t give PhD degrees as they are autonomous institutes and not affiliated to universities. People usually say, there is no big difference between MBA and PGDBA or PGDBM. It is a marketing tip to sell the seats available in their institutions. But, there is 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 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 on a executive or Sr. level managers. PGDM is tactical and MBA is strategic.
In US they call PGDM as PMP.