A hypervisor is the software layer between hardware and several operating systems that gives each virtual machine its own isolated execution environment. MSRC's Hyper-V bounty table splits guest escapes into L1 (up to $250,000) and L2 (up to $150,000); the difference is which wall gets crossed.
A VM expands the process interface into a machine interface; the hypervisor directly executes ordinary instructions, traps privileged operations, and virtualizes interrupts, I/O, and two-stage address translation.
OpenStack combines Keystone, Nova, Neutron, Glance, Placement, Cinder, and other services into multi-tenant IaaS; adopting it means staffing a cloud platform team, not finishing an installation.
Proxmox VE integrates VMs, containers, clusters, HA, storage, and backup; it simplifies virtualization management while hardware, quorum, networks, capacity, and DR remain yours.