why would there be such a difference between face hugger and the xeno?

I've never thought of that before.The facehugger plays an integral part in the xenomorph life cycle yet shares almost no physical qualities with it's spawn.It could be related to it's sacrifice.

This crossed my mind a lot, and i cant really think of a reason for it....

The Face Hugger and Egg are not really Bio-Mechanical not like the Alien that came out of Kane...

But each movie since... includes the Queens... they never quite appear as Bio-Mechanical as the 1979 Big Chap... maybe we would get the reason for this some day?

Maybe they have changed the Aesthetics so the Xeno is now not as Mechanical as the Giger Concept and Alien in the 1979 movie?

Trying to think of a answer.... maybe Kane is not 100% Human lol

It's a completely unique life cycle, to be sure. The facehugger is perfectly suited to its purpose. To impregnate life forms with easily identifiable heads with whatever makes the Xeno. 

This is where the life cycle breaks down, logically. In every species I can think of, it's the adults that lay eggs or engage in reproductive activities (by adult I mean reproductively viable, not 18 years old).

maybe the facehugger is needed as the dominant gene/DNA (carrier) so that it can get its full potential out of any host it attaches to.

"why would there be such a difference between face hugger and the xeno?"

Why would there be such a difference between a sperm and egg and a fully developed human?

S.M. not exactly an apples for apples comparison. Neither of the things you mentioned are separate organisms. It is also worth mentioning that an egg does not come from sperm and sperm is not deposited by eggs. 

The facehugger is literally a completely different animal that asexually implants the Xeno minutes or seconds postnatal. It is the only thing that doesn't fit. What other Animal is capable of impregnating anything seconds after it's born?

The hugger is, as described by its creator, an "ambulatory penis".  Looking at it that way, rather than a separate animal, makes the analogy a bit more apt.

If you look at the facehugger closely, you can assume enough from it's skeletal structure and segmented tail to get the bio-mech. I'd be surprised if it wasn't originally designed this way too. The facehugger we see in Alien looks more mechanical than the original sketches done by Giger.

Maybe the embryo was able to utilize a synthetic's fluids to develop but evolve like it seems to have done with any other being. (This is if a synthetic was face hugged- which we don't have evidence happened as of yet).