I'm going to clear up the confusion in your question. Firstly, IVs are hidden values that you cannot "train" as such. Once you have a Pokemon the values are fixed. EVs are the ones you can train - you start with 0 and gain them as you battle. The idea is to try and gain specific EVs in stats, e.g. Attack and Speed.
The Power items have a couple of different uses. As a hold item when BATTLING, the Pokemon gains 4 EVs in a particular stat, plus the EVs the Pokemon you're battling gives.
As a hold item in DAYCARE, the Pokemon passes down its IV stat to the baby. This is the only way to influence what IVs a Pokemon has. Once they have hatched from the egg you will need to EV train the Pokemon.
Pokemon that are left in daycare to just level up do not gain EVs and obviously their IVs never change.