Xen comes in 2 flavors but can run simultaneously on the same physical host, Xen PV (paravirtualisation) and HVM (full hardware virtualisation)
Xen PV guests (in the hosting industry) tend to be template based for repid deployment and snappy performance, you can run your own kernel in Xen PV and this is pretty much default these days, you can only run Linux on Xen PV (BSD with additional configuration is possible but not common).
Xen HVM runs much like KVM it has better drivers for Linux based distributions as PV has been available by default since around 2006 in most kernels so you do not need to install virtio for a performance boost however NetBSD and windows perform poorly on Xen HVM compared to KVM, while you can over come this to some degree on Windows with Xen PV drivers for Windows it does not run as well as KVM out of the box so to speak.
Xen is quite old now and very mature, most people select Xen for good performance with exceptional stability.
Xen hosts will usually pre-allocate Ram and CPU cores to the xen hypervisor so it has its own dedicated resources that guests cannot impact on to achieve stability.