There is a second network interface attached to each pod and that interfaces are connected to vxlan-based overlay network. So during the migration VM can keep the same ip address (APR will take care of traffic finding it's destination). That is simplest approach but has it's own downsides when overlay network grows big and painful to set up with some cloud SNI.

Few other options are:

* use some SDN instead of vxlan

* use QUICK instead of TCP in the internal network -- with QUICK we can change endpoint IP addresses on live connection