If there a lot of pro of last KVM versions vs OpenVz then good for KVM, but is there a migration tool from OpenVz to KVM vms?
no, see http://pve.proxmox.com/wiki/Migration_of_servers_to_Proxmox_VE
as OpenVZ has no Kernel, you cannot migrate automatically to KVM - basically you need a running KVM (with the same operation system) and then you can move the container files to this new machine - including a lot of manual work.