VM has gone through several forms in it's 30 year history. Melinda Varian's VM History paper details the process and development in great detail, however here is a quick recap of the early forms of VM:
CP-40: Developed in the early 1960's to virtualize the 360/67 before real hardware was available.
CP-67: virtualized the 360/67 and early 370 hardware.
VM/370: First "official" external release of VM to customers.
VM/SP 1: updates and new products
VM/SP 2: more updates
VM/SP 3: more updates
VM/SP 4: first appearance of GCS and REXX in the base product.
VM/SP 5: more updates, CMS Windows, the beginning of few OCO modules.
VM/SP 6: Last 24-bit only release.
There was a variation for SP 3 through SP 6 -- the High Performance Option (HPO) that added a variant scheduler, swapping as well as dynamic paging, and support for memory > 16M on certain machines (30x0 processors).
VM/XA MF: First generation 31-bit VM. Very short lived. First appearance of XA I/O subsystem and IOCP.
VM/XA SP 1: bimodal (24/31 bit CMS). Very short lived.
VM/XA SP 2: Last release of VM/XA. maintenance release, and significant for forced use of VM/SES.
VM/ESA 370 feature: 1st ESA architecture release. 370 feature for non-ESA architecture machines. Interim release. Last VM release for non-ESA capable CPUs.
VM/ESA ESA feature: 1st ESA exploitative release. Released to support new features on ES9000 hardware. Interim release.
VM/ESA 1.1: Converged release for ESA systems. Dropped standard I/O model and required an ESA capable CPU.
VM/ESA 2.x: Continued evolution, introduction of CMS Pipelines, bundling of TCPIP, RSCS, TSM/VM products as preloaded.
VM/ESA 2.4: Last VM/ESA release.
z/VM 3.x: First z/VM release. Last release to run on pre-9672 G5 hardware. Last 31-bit only release. Supported IFL processors with PTF. Last release of only MMC-based pricing.
z/VM 4.x: First 64-bit capable release. Full support for IFL processors. First VM release to require G5 (ALS2) hardware. TCPIP support bundled into base product.
z/VM 4.2: Introduction of guest LAN support. Significant enhancement to TCP suite. Introduction of SSL support. Introduction of flat rate per-engine pricing model.
z/VM 4.3: Support of broadcast and multicast on guest LANs.