VM/370 was the first operating system on the IBM System/370 mainframe platform that supported 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.
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
31bits and beyond
There has been a movement to expand the 370 virtual machine to incorporate 31bit addressing of the 390, allowing larger program sizes. Primarily this has been to allow gcc to self host under MVS.
There has been regular updates to what is currently known as the 'six pack'.