Who are the Time Keepers?
As explained by Miss Minutes in a video Loki was forced to watch before facing trial for committing a time crime, the Time Keepers came into existence after "a vast multiversal war" involving various timelines almost wiped out reality. These all-powerful beings reorganized the multiverse into one sacred timeline. They now protect the proper flow of time for everyone and everything.
In the comics, the Time Keepers wield almost limitless powers over time manipulation and are guardians over the stream of time. They're not infallible and are also willing to do whatever it takes to preserve their existence, which has pitted them against various teams such as the Avengers.
What is the Time Variance Authority?
As explained in "Loki," the Time Variance Authority is an organization created by the Time Keepers to uphold the sanctity of the timeline and all reality. Its job is to make sure that the one true "sacred timeline" remains intact by nipping any possible branches that sprout from establishing alternate timelines and creating alternate universes. The TVA itself exists outside of the flow of time.
The TVA made its first comic book appearance in "Thor" in the 1980s. Similar to its onscreen incarnation, the TVA in comics monitors the multiverse and erases any timelines that are perceived to be too dangerous. The bureaucratic agency also steps in to stop those who try to meddle with the past or future, though that doesn't mean it always prevents everyone from time travel or altering reality.
In the comics, the final chairman of the Time Variance Authority created the Time Keepers.