vSphere Hypervisor (ESXi) — VMware’s bare-metal layer What it is ESXi, officially called vSphere Hypervisor, is VMware’s bare-metal hypervisor. You install it straight on the server, no Linux or Windows host underneath. For many datacenters it’s the base layer: stable, predictable, and designed to integrate with the rest of VMware’s stack (vCenter, vMotion, HA, DRS). Free to download, but the good stuff (central management, clustering, API writes) comes only with licenses.

oVirt — upstream virtualization for Red Hat shops What it is oVirt is the community project behind Red Hat Virtualization (RHV). At the core it’s just KVM + libvirt, but with a full management layer wrapped around it. Think of it as “vCenter, but open source and Red Hat flavored.” It’s heavier than Proxmox, but it has been running in production datacenters for years where admins wanted VMware-like features without VMware licensing.

Xen Project — bare-metal hypervisor that refuses to die What it is Xen Project is a type-1 hypervisor that’s been in use for more than 20 years. It started as an academic experiment in Cambridge, later became the base for many VPS platforms, and even powered AWS EC2 for years. Today it’s still maintained under the Linux Foundation. Not as trendy as KVM, but still useful if you need a small, security-focused hypervisor or want to separate workloads at the hardware level.

Windows Sandbox — disposable Windows environment for safe testing What it is Windows Sandbox is a feature built into modern Windows (Pro and Enterprise editions, from Windows 10 1903 onward). It provides a temporary, isolated environment that runs a clean Windows instance on demand. The main idea is simple: you launch Sandbox, test or run something risky, then close it — and everything inside is discarded. No need to spin up a full VM manually or reinstall the OS after running untrusted software

Virtuozzo — mix of containers and VMs with hosting DNA What it is Virtuozzo has its roots in OpenVZ, the old Linux container project. Over time it turned into a commercial platform used by hosting providers to sell VPS long before Docker became a buzzword. Today it combines two approaches: lightweight containers for density and KVM-based virtual machines when you need a full kernel. That mix is what made it popular in the service-provider world.

VirtualBox — cross-platform workhorse for labs and odd jobs What it is (short) VirtualBox is a type-2 hypervisor maintained by Oracle. It runs on Windows, Linux, macOS, and Solaris, and it spins up guest systems with minimal ceremony. Not the fastest thing on earth, and that’s fine; it’s the “it works everywhere” option that sticks around in classrooms, dev laptops, and compatibility test rigs.