So when you live migrate a VM, Xen will decrease memory dynamically to the minimum. If you didn't plan correctly dynamic min to a value that's OK for your system, you might have memory problems for your operating system (eg if you system can't survive with less than 4GiB RAM and the dynamic min is set to 3GiB, this will cause problems!). So planning is important.
There's some edge cases also in migration when you have to tell Xen about memory management when you grow/reduce amount of RAM. But it works generally OK if you took care of dynamic min as I said.
