As mentioned by others, xen is used a lot in the wild (I believe it is the underlying virtualization architecture on EC2, for example).
One big advantage of KVM is that you can use your existing kernel, and there is little chance of conflict with other features in the kernel (which can include somehow esoteric drivers, more fundametal patches like low-latency stuff, new schedulers, etc...). At my current employment, we had issues with xen and raid drivers for example which did not work with the xen kernel (but that may be our own incompetence).
When KVM was first out, it required support from the CPU, whereas xen did not. KVM still requires this CPU support, but it is very hard to buy a x86 CPU without the feature nowadays, whereas it was not so common 3-4 years ago. Anything you would use in a server certainly has the support.
KVM is simpler to set up on a common distribution (Ubuntu, RHEL, Fedora), which means it is easier to reproduce the same environment as used in production.
One big advantage of KVM is that you can use your existing kernel, and there is little chance of conflict with other features in the kernel (which can include somehow esoteric drivers, more fundametal patches like low-latency stuff, new schedulers, etc...). At my current employment, we had issues with xen and raid drivers for example which did not work with the xen kernel (but that may be our own incompetence).
When KVM was first out, it required support from the CPU, whereas xen did not. KVM still requires this CPU support, but it is very hard to buy a x86 CPU without the feature nowadays, whereas it was not so common 3-4 years ago. Anything you would use in a server certainly has the support.
KVM is simpler to set up on a common distribution (Ubuntu, RHEL, Fedora), which means it is easier to reproduce the same environment as used in production.