Once upon a time, there was nothing but native, or bare metal, hypervisors (a.k.a. virtual machine managers). In the 1980s, I cut my teeth on IBM System/370 mainframes running VM/CMS, but bare metal's history goes all the way back to the 1960s. With bare metal hypervisors, the hypervisor runs directly on the hardware. There is no intervening operating system.
The formal definition of bare metal hypervisor, or, as it was called in its day, Type 1 hypervisor, goes back to Gerald Popek and Robert Goldberg's seminal paper, Formal requirements for virtualizable third generation architectures. They also defined bare metal's great competitor, the Type 2, or hosted hypervisor.
Today, bare metal virtual machines are still very much with us. VM/CMS evolved into IBM's z/VM. And there are many other bare metal systems. Chances are you and your crew are using one even now.
While the implementations are quite different, the name of the game is to provide a minimal operating system that provides just what's needed to run virtual machines. No more, no less. If you have an extra layer, like an operating system, between your VM and the hardware, that opens the door to performance, latency, security, scalability, and VM isolation problems.
There are corner cases of course. For example, you can still get a hot argument going in some circles if you suggest that KVM isn't really a bare metal hypervisor.
For your servers, whether it's just one Xeon box in the server closet, a thousand servers in your data center, or ten-thousand in your private cloud, what you really want is a bare metal hypervisor.
How to choose
For the mainstream operating systems there are four main choices: KVM, ESX/ESXi, Hyper-V, and Xen, in one form or another. You can argue until you're blue in the face about which one is "better," but generally speaking they all do an excellent job. More to the point of justifying the purchase to your CFO, each has its own role to play.
Which role depends largely on your other choices in platforms and operating systems.
From my perspective, here's where each is best suited for deployment.
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