Is Violet Evergarden a robot, a cyborg or a human?
She is human. This can be further proven by the fact that:


She can bleed. As seen during the flashbacks of the war from several episodes, namely one and two. If it was oil, it would've been black; which it wasn't. 
She has emotions. She was worried about Major Gilbert and was curious about the meaning of the phrase, 'I love you' and how it made her feel. 


She only looks like a robot because of her mechanical arms. She lost her human arms during the war. She is only awkward, because she never understood how to interact with other individuals in a normal society. She only knows how to act like a soldier.