The precociously well-read Kevin knows a lot of stuff about language, history, and mythology, to name but three subjects, and he proceeds to teach Max. One of the things he teaches Max is the ancient legend of King Arthur, the British king who allegedly established his kingly credentials when a mere youth by pulling the sword Excalibur out of a stone.
The legend of King Arthur has really captured Kevin's imagination, as it has captured the imaginations of many down the centuries. To a large extent, this is because he regards knights as being the first robots, another of his obsessions.
Kevin's fascination with the legend explains why he refers to his mother Gwen as "The Fair Guinevere." Max, who's blissfully ignorant of Arthurian legend, thinks that Kevin's saying "Fair Gwen of Air," which makes absolutely no sense to him at all. But Kevin dispels his ignorance, telling Max that he's referring to "Fair Guinevere" from the legend of King Arthur.
In the legend, Guinevere was Arthur's wife and queen. She was also an adulteress, who had a torrid love affair with Sir Lancelot, one of the Knights of the Round Table. This makes her comparison to Kevin's mom not entirely flattering. Thankfully, however, she doesn't seem to mind.