OpenVZ is an OS-level virtualization technology and relies on the host node kernel. KVM is true virtualization where VPS operates as it's own server.
Since OpenVZ is an OS-level virtualization, it consumes far less resources than a KVM based VPS and OpenVZ VPS should perform better due to less overhead.
OpenVZ can only run Linux distributions while KVM can run any operating system. OpenVZ has lower management complexity for customers than KVM.
If you need any assistance choosing the right virtualization for your needs, please contact us.