There are a few tools, but to get you the right ones we need to know from what system to what system. Is it Hyper-V to VMware?
This is not for a one-off project. We would like to be able to switch hypervisors from time to time, for instance developing in HyperV but then moving to ESX when we are ready to go into production. Also we don’t want to be held hostage to one hypervisor.