Guinevere is best known for her role in toppling Camelot by her affairs with other men. Originally this was not with Lancelot (he did not exist in the early chronicles). Chretien de Troyes gave us the Lancelot-Guinevere adultery story: when he invented Lancelot and added him to Arthur's court as a Knight of the Round Table.
Guinevere appears in the earliest stories of Arthur as his faithful wife. The Welsh Triad 56 names the three queens of Arthur, all three named Gwenhwyfar. It is widely accepted that the famous Gwenhwyfar of legend is the third one mentioned; she is the "daughter of Gogfran the Giant."
Geoffrey of Monmouth portrays Guinevere as a noble lady with a Roman heritage, raised by Duke Cador of Cornwall. She is not mentioned much again until the end of Arthur's reign when she betrays the King by becoming the Mordred's lover. She runs away to Caerleon and enters a convent. The recurring theme of her love affairs, whether by consent or abduction, has many variations. Early stories give Arthur as her rescuer, while later the rescuer is Lancelot.
Giraldus Cambrensis says that there was an inscribed cross on the the royal grave at Glastonbury which named Guinevere as Arthur's second wife. However there is only one surviving drawing of the cross which only shows one side of the cross, and this does not mention Guinevere. So it is not possible to verify Giraldus's version. If you accept that Arthur was buried at Glastonbury, then you would probably accept that Guinevere was buried with him.
The legends would change over time, and her rescuer would become Lancelot and there would be a Love Triangle between Arthur, Lancelot and Guinevere. Lancelot is entirely the creation of Chretien de Troyes and only appears at this date.
Other writers have tended to follow this theme. Sir Thomas Malory's version portrays Arthur's ignorance or perhaps denial of the affair, as having undermined Arthur's authority with his knights. Malory also introduced the story of Arthur finding his queen guilty of treason and sentencing her to death by burning at the stake. Lancelot then rescues her, the battle between Arthur and Lancelot then ensues. Like Geoffrey, Malory tells of Guinevere retreating to a nunnery in the end.
Tennyson finds the adultery to be the cause of all that is wrong with Arthur's court. Because of his sin, Lancelot cannot behold the full glory of the Holy Grail. Modern writers would treat the adultery as a matter of course and even suggest that it was inevitable becaue Guinevere didn't really love Arthur. She enters the convent at Amesbury after the final battle. Lancelot finds her there but she sends him away with a penitent farewell, realizing that their deeds have brought about ruin of Camelot.
So various sites for Guinever's final resting place are Glastonbury, Meigle in mid-Scotland north of Perth, Caerleon in Wales.
Full references to more about Queen Guinevere at the Rochester site
Characters from King Arthur's legends