Initial margin posted under the margin rules generally cannot be reused and earns no investment benefit. The cost of funding it is therefore always a cost that lowers the value.
Consider a hypothetical example in which the expected initial margin at some future date is 40 million yen and the funding spread is 0.50% a year. The cost for that year is 200,000 yen (before discounting). MVA is the sum of these amounts, discounted, over the life of the trade.
Exchanging initial margin reduces CVA and KVA but creates MVA. Which effect is larger depends on the funding spread and the counterparty's rating.