OpenVZ is an OS level virtualization. The OS is partitioned and assigned resources, so the plans show two resources viz. dedicated and burst. The dedicated are the guaranteed resources to the server and burst are the remaining which are unused by the complete system and released to your server when required ( on load ).
So a single OpenVZ VPS is actually running on resources which are shared with another VPS box. Mostly the burstable resource is the RAM which is borrowed from another VPS box when its not in use and your VPS box needs it.
On the other hand, KVM is hardware virtualization. The main OS simulates hardware for another OS to run on top of it. It fairly distributes all the resources. It does not have burst resources but it guarantees the dedicated resources. Essentially this means the plan which you see its 100% with you and is never shared with anyone. This also means on load you will not get extra ( burstable ) RAM out of it.
Guaranteed streamline performance is observed in KVM but value per dollar and efficient use of the complete hardware is observed in OpenVZ.
tl;dr :
OpenVZ is OS virtualization. Resources are variable and are shared with other(few) users on the same server as and when required. Dedicated resources are guaranteed but they could be shared with others and your box can borrow when others are idle.
KVM is hardware virtualization. All the resources are dedicated to you and only you. None is shared nor you get the shared bonus if required to your server. Dedicated resources are guaranteed.
