As Jonathan Hobbs points out, a full-round action takes up your whole round, but you can spend a standard action to start or complete selected full-round actions. Not all full-round actions are available, and the caveat is that you have to complete it on the following round (not in the same round).
Start/Complete Full-Round Action
The "start full-round action" standard action lets you start
undertaking a full-round action, which you can complete in the
following round by using another standard action. You can’t use this
action to start or complete a full attack, charge, run, or withdraw.
The restriction on attacking, charging, running and withdrawing makes this of limited use, but if you could maintain the state of having three (or more) standard actions over multiple rounds, then you could use your last action to start a full round action, finish it with your first standard action of the next round, then use your "spare" standard actions on whatever else, giving you a full-round action per round, plus standard actions for other things.
Going beyond the RAW, this does somewhat imply that full-round actions are roughly equivalent to two standard actions.