Here in the US, most slaughterhouses use one of
these.
You calmly walk up to the cow, quick zoop this thing right between its eyes on the top of the head, pull the trigger, and blam, it is unconscious forever. It is then hoisted into the air (very carefully, making sure not to slam it into anything so it bruises) by its back legs, tied together, sometimes around the hooves, sometimes through the leg tendons and whatnot, then its jugular is cut, allowing it to bleed out.
You can repeat the above for pigs.
As for chickens, there are shock/stun baths, usually. It's an electrified pool of water that intends to shock the bird unconscious, then it moves into a pool of very hot water, a scald bath, to loosen the feathers.
However, this method is not particularly good. Sometimes the bird is not stunned into unconsciousness by the shock bath. Sometimes the bird is shocked previous to the bath through conductive means, resulting in seizures pre-bath. Sometimes the bird is moved into the scald bath while still alive.
When the animal becomes stressed from struggling or harm (physical or mental), it begins to build up lactic acid in its muscles. This turns the taste from standard to awful. An animal with high lactic acid is not edible in extreme cases. An animal that has been bruised does not have a usable hide.