My main reason not to have Xen is that I do quite a bit of staging on my laptop. And Xen, however easy it is to get running on Debian, is practically a new architecture by itself - At least, architectural variant. I want my laptop to have ACPI support - and there is no way to achieve that with Xen. KVM "just works", and it just does not bother me when I'm not using it.
And BTW, ACPI is not only a good feature for laptops. I want my servers also to suck less energy when demand drops at night.