Yes, you can over-subscribe RAM either through ballooning or just by over-subscribing the host.
Both KVM and Xen use QEMU for their device models so the machine they emulate is very similar (at least in HVM mode). Xen and KVM use different paravirtual I/O drivers but everything I've ever seen indicates that I/O performance is better in KVM.
Xen PV guests are a mixed bag. On the one hand, they work on boxes without virtualization hardware (which are rare anymore) but on pre-Nehalem/Barcelona processors, they are much slower than using HVM (especially 64-bit).
I've never seen a good Xen HVM benchmark either comparatively speaking... Of course, I'm more than a little biased :-)
Both KVM and Xen use QEMU for their device models so the machine they emulate is very similar (at least in HVM mode). Xen and KVM use different paravirtual I/O drivers but everything I've ever seen indicates that I/O performance is better in KVM.
Xen PV guests are a mixed bag. On the one hand, they work on boxes without virtualization hardware (which are rare anymore) but on pre-Nehalem/Barcelona processors, they are much slower than using HVM (especially 64-bit).
I've never seen a good Xen HVM benchmark either comparatively speaking... Of course, I'm more than a little biased :-)