VM/370
From Computer History Wiki
VM/370 was the first OS on the S/370 mainframe platform that facilitated the creation of Virtual machines. This was a VERY popular OS for people to load onto their mainframes as now they wouldn't have to choose which OS to load, for applications as they could now load ALL of them.
Documentation sets
Bitsavers has a few manuals online. They can be found here:
Getting this to run
Thankfully the software is available, although I've found a few 'issues' with the install process and I've outlayed a tested one here: Installing VM/370 on Hercules