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I'm experimenting with Xen on a spare machine and I noticed that it can resize the RAM and CPU allocations of my VMs without shutting them down first. VirtualBox, VMware Fusion, etc. can't do the same thing. How does Xen live resize work?

(Does it have anything to do with my guest operating systems using paravirtualized kernels?)

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VMware can doo this too. So can KVM. For those it has to be enabled before the machine boots and is called memory ballooning. Xen does it withou, as it is (as you mentioned) paravirtualized. Same applies to OpenVZ. – zhenech Mar 22 '14 at 21:19
Both Windows and Linux support hot adding hardware if you have the right kind of physical equipment. To do it in a VM simply requires emulating the correct correct equipment. Windows has supported this since Win 2003. – Zoredache Mar 23 '14 at 7:22

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