Current leading enterprise-level virtualizations are

• Redhat KVM
• VMware vSphere
• Microsoft HyperV
• Citrix XenServer

All of them have many successful cases deployed in IT infrastructure. VMware vSphere has a rich experience in live migration while HyperV just released their share-nothing live migration in Oct 2012.

To migrate a virtual machine, there are different states that could be transferred over between servers:

• CPU state
• Memory state
• VM image

While services are running inside a vritual machine, hypervisor needs to keep syncing the state changes between the source and destination servers when does the live migration.

In general, there are two different types of live migration to each of Virtualization technologies: memory migration and storage migration:

• Memory migration: CPU and Memory states are migrated but VM image stays on shared datastore.
  For memory migration, each virtualization requires VM's image hosted on a shared storage like NFS or Samba instead of local disk so that even VM instance is running on a different server but VM is still able to access the remote VM image. CPU and Server architecture needs to match on both source and destination server.
• Storage migration: VM image is also migrated from source datastore to destination datastore.
  For storage migration, VM image can be hosted on a local disk and VM image needs to be migrated to the destination local disk when storage migration happens. But for source and destination server needs to have the same type of file system and same file folder for hosting VM image.