PIM, PAM and PUM have different meanings, and interpretations, to different people. For the most part the concepts around these three far-ranging topics intersect, and for the most part we are talking about the same thing. PIM, privileged identity management; PUM, privileged user management; and PAM, privileged account management OR privileged access management.
All three of these acronyms revolve around a few simple concepts: who can get to a server, virtual machine or system image, how they can get into the operating system and what they can do when they get there.