KVM and OVZ are types of virtulisation with servers. Server hosting companies have what is called dedicated servers, very powerfull machines. These dedicated servers are split up into VPS’s which are smaller servers which can have a different amount of resources as split from the dedicated server.
There are many types of virtulisation but in a sense its either KVM or OVZ in the end.
To keep it short and simple:
OVZ can only run linux OS, can have many more instances from an equivalent KVM setup, best scalability and is light weight.
KVM is slightly lower performance, all OS possible (windows), unlikely to be oversold and is a full virtualization.
OVZ is prone to being oversold by hosting companies so be aware of this.