You explain (pretty well :) several uses of *virtualization on the desktop* (the VM runs in the user system). Maybe it worth noting that this is rather different from what is generally understood by *desktop virtualization* (the VM runs in servers and remote protocols are used to access the desktop from the user system). »

Errata: [...] What virtualization actually means is the decoupling of logical resources from physical infrastructure, no matter if this decoupling is N:1, 1:N or even 1:1 :) »

Regarding performance, I don't have the benchmarks now, but in my humble opinion and based in my experience with VMware, Xen and User Mode Linux, I think that a long way has walked since late 90s, when VMware launched its first products to virtualize x86 platforms. Current techniques (specially hypervisor-based ones, such as Xen or VMware ESX, etc.) are highly optimized so they can reach a performance ratio quite close to native one. Specially when paravirtualized OS or the hardware extensions provided by the main CPU vendors (Intel VT-x and AMD-v) are used. Your las paragraph also bring an interesting point. Virtualization doesn't necessarily means to split a physical machine into multiple virtual machines, but also to aggregate the combined power of several physical machines into one big virtual machine. What virtualization actually means the decoupling logical resources from physical infrastructure, no matter if this decoupling is N:1, N:1 or even 1:1. Have a look to PVM, for example. »

Absolutely right :) The operating system is more than the kernel, so my statement should be "all these environments share the same _kernel_", as you said. Operating systems (each one contained in a isolated root filesystem in the hosting machine) can vary. I know a bit about FreeBSD and Linux Vservers and they behave the same way (i.e., same kernel, different per-filesystem operating systems). »

Very good article! However, some litte comments... You said "all virtual machines must run the same operating system [using paravirtualization]". I should say this is not necessarily true, at least for Xen. Each domU (the term to refer to virtual machines in Xen parlance) can use a different kernel version (you set it with the 'kernel' parameter in the domU configuration file). Beyond the kernel, each domU can use a different filesystem (i.e., virtual disk), set with the 'disk' parameter. In addition, I should add another kind of virtualization to the list, usually knows as system-level or operating system virtualization. Some examples are Linux Vservers, FreeBSD jails and OpenVZ/Virtuozzo (the one you mentioned! :). Some people consider they aren't a true forms virtualization, but the true it's that they provide isolated execution environment in a lighter way than the other approaches (hardware virtualization, either application or hypervisor based, and paravirtualization). And, in this case, all these environments share the same operating system. »