Virtualization and resources
OpenVZ
Container virtualization: cheaper, but the kernel is shared with neighbours — not all software will run.
OpenVZ is containers on a shared Linux kernel. All VPSes on the server run on one kernel, so they can be packed denser and sold cheaper than KVM.
The limits are noticeable: no Windows and no custom kernel, Docker and some VPNs often do not work, memory may be shared with neighbours. Fine for a simple site or bot; for the rest, KVM is better.