Yes, all the films, including The Hateful Eight, are a part of the same cinematic universe.
In an interview released just seventeen hours ago, Tarantino clarified that the films Kill Bill and From Dusk Til Dawn are the film within the films from the "Real Universe" that the characters watch. The "Real Universe" is the cinematic world where everything takes place in the same setting.
According to Tarantino, in the article published by Dazed Digital:
There is actually two separate universes. There is the realer than
real universe, alright, and all the characters inhabit that one. But
then there's this movie universe. So From Dusk Till Dawn, Kill Bill,
they all take place in this special movie universe. So when all the
characters of Reservoir Dogs or Pulp Fiction, when they go to the
movies, Kill Bill is what they go to see. From Dusk Till Dawn is what
In an updated version of that same article, Tarantino confirmed that at least one of the characters in The Hateful Eight is in fact related to a character from a previous film, which takes place further along the timeline.
I don’t want to say it point blank ‘boom’, but if you’re a fan of my
work and are familiar with it you could be a bit of a detective. I
will say that there is one character amongst the eight that is related
to one of my other characters in one of my other films.
As for the identity, Tarantino has not affirmatively confirmed, but the strongest clues hint at Tim Roth's Oswaldo Mobray, aka Pete Hicox, who shares the same legal name with Michael Fassbender's Archie Hicox in Inglourious Basterds.