[fedora-virt] is there a best clock to use?

Is there a HOWTO anywhere of the different ways to make time of day clock work in virtual machines and different ways to influence it from the qemu side of the world? Time keeping is horrible in most of my virtual machines, and I just wonder if there are settings I can change in libvirt or kernel boot parameters I can apply in the linux guest to make things slightly less horrible? Sometimes I get the impression that the guest time is already two or three minutes off as soon as it is booted, and I can't imagine how that happens since I would assume it gets a copy of the host machine time for the emulated rtc clock in the guest. How can it already be broken at boot time?