I'm pretty sure Xen on ARM has been a thing longer than KVM on ARM.

But in general, why Xen over KVM? the primary reason is that I understand xen better.

I have a secondary reason, and it's a long-term bet against KVM.

KVM has a lot of features that while they are interesting in the corporate space, make oversubscription too easy in the hosting space. Set up a KVM host with a bunch of ram and a bunch of swap, and start handing out "memory" to your kvm guests? By default it will get ram or swap as availability and usage dictate, just like any other process.

To be absolutely clear, as far as I can tell, none of the current KVM VPS providers are using these over-subscription methods right now. I'm not accusing anyone of anything.

However, the primary reason why Xen beat OpenVZ and the other containerization systems was that it is really quite difficult to oversubscribe ram on a Xen host, and your pagecache was in your ram. It was not shared with others.

Sure, if the manager of an OpenVZ host allocates resources responsibly, it can be more efficient than Xen. And some hosts did that. But, because some hosts did not do that, Xen developed a reputation. Buy a xen host, and you are getting honest ram. Buy an OpenVZ host and you are relying on the reputation of the host.

KVM, now, can be configured to run in the same way Xen is, with no sharing of memory, pagecache or otherwise.

However, it is simple to give KVM guests memory that is ram /or/ swap, and other than performance, it would be impossible to tell, from the guest, what I gave you.

Now, of course, right now KVM enjoys the same reputation as Xen, because all the current KVM providers seem to be doing the responsible thing and only handing out ram, not ram/swap mix, but at some point? someone is going to change that.

The irritating thing is that the xen developers seem to be running as hard as they can to put in kvm-stlye features to share more memory. Isolation, when you are in a multi-tenant environment, is often more important than overall efficiency.

You can use either. Both KVM and Xen developers have had this hardware available for quite some time, and before that we'd been developing on the Foundation Model (very slow) simulator for over a year.

By the time this hardware is widely available, all your favourite Linux distros will Just Work, with ~99% of the functionality/packages/languages of x86. In fact that's pretty much the case already.