IBM VM is a wonderful operating system, both as a hypervisor (creating guest environments) or as an operational environment. We had both. VM/ESA on the machine, then an operational VM/XA for business users and MVS/ESA sharing the same physical mainframe. We switched it off in the early hours of 1/1/2000, although it still worked just fine.
There are lots of links easy to find about it:-
Let me google that for you
Kind regards (and with fond memories)
Robin