Species: A group of Organisms capable of producing fertile offspring
Well that just fucks this up doesn't it?
A facehugger can produce fertile offspring with anything (As far as we know)
So in theory, We all belong to the same species as Xenomorphs. Right?
Wrong. We (probably) share genetic material so dissimilar that we can't call them the same species. Despite our ability to fuck.
What about Parasites?
Although we consider a chestburster a parasite (Which is in some aspect true) no known parasite actually takes genetic data from its host. A parasite can't produce fertile offspring with its host, It simply lives inside it. Unlike the Xenomorph. (See the dog Xeno)
So if they aren't a species, What are they?
They are Simply alive. Just because it is alive does not mean it has to fall into our system.
It may be alive, But that doesn't make it a species. The word species is currently only used to describe life that originated on earth.
/science