If it's fine on Xen, but not KVM, then it's a host problem. The application doesn't know (or care) what it's on - the speed depends on how fast it can process, and how fast it can get to MySQL and how fast it can process.
I prefer Xen over KVM, esp in production environments, it's a lot more stable, and behaves more like an actual dedicated box. KVM chews through resources pretty fast, and if that host has a lot of nodes on the host (which a lot of hosts do, same as shared hosting), then there will be the slow performance that you're seeing.
Contact your host and tell them that it's slow. SSH into the phpVMS server and do a ping to MySQL, also look at the CPU using through htop. Most likely, it's that the node is over saturated and doesn't have the CPU to work as fast.