We are using highly modified OpenVZ virtualization platform. For each VPS, we allocate dedicated RAM, disk space and network resources without sharing it with another container in same node. There is no “burstable”, “flexible” or other non-guaranteed RAM allocation method.
In KVM virtualization besides OpenVZ dedicated resources we also dedicate CPU for full utilization at any time. More information of a difference between OpenVZ and KVM can be found here.
Linux VPS offers kernel version of 3.X+ and higher.