The answer: 4.
1. Failover Cluster Manager
2. Move-ClusterVirtualMachineRole PowerShell cmdlet (part of the failover clustering cmdlets)
3. System Center Virtual Machine Manager 2008 R2
4. Move-VM PowerShell cmdlet (part of the SCVMM 2008 R2 cmdlets)
At the end of the day, how you perform a Live Migration is going to depend on how you like to work – GUI or command line, and then it's down to preference under those. Not everyone will deploy SCVMM, so what you get natively with Windows Server 2008 R2 in Failover Cluster Manager and its PowerShell cmdlets.