Linux should be straight forward (but there is no special tool for this) as OpenVZ is the basis of virtuozzo for linux.
As Virtuozzo for Windows is a quite different from all other full virtualization systems like KVM this will be more like a reinstalling ... but never tried and depends what is running on win. If you find a tool for migration to vmware, then you can use this also for KVM.
has anyone maybe migrate windows from virtuozzo to proxmox? I need to migrate some servers and i would like to know if someone has some experiences whats the best practice?