Kendra Saunders is the grandniece of Shiera Hall, the original, Golden Age Hawkgirl. Due what I call "Johnisms", Kendra was introduced as having committed suicide, BUT the soul of her great aunt (Sheira) "possessed" and remained in her body. Johns is a big fan of the Golden Age Hawks (Carter and Shiera) but for some reason didn't want to revive either fully, until Brightest Day. Kendra (possessing Sheira's soul) had all the same memories and experiences as herself and non of the memories of Shiera. You ask your self, why bother with the soul possessing thing? Johns did a similar thing with Hawkman, Dr. Fate, and when he revived the original Secret Society of Super Villains. He does these extravagant things, that only matter for one stretch of story (usually 6 issues) and then stops talking about/ignoring what he instituted. Normally, when things become convoluted and don't work out the way he planned.
Here are some pictures and mini bios to help you discern which Hawkgirl is which.
This is Sheia. Wife of Carter Hall, former archealogist, and eternally locked with her husband in a cycle of death and rebirth. Stemming from being the reincarnations of Egyptian prince and princess, Khufu and Chay-Ara. She is the main Hawkgirl, and the one DC exposes the most. She got her start in the Golden Age, and has endured despite COIE and Zero Hour seemingly wiping her continuities from existence. She got the most exposure on the Justice League show from a decade ago.
This is Shayera Thal. She is from Thanagar and is a police officer on the planet. Her partner (police partner) is called Katar Hol. She was the Silver Age Hawkwoman, but the most distinct version is from the 1989 series "Hawkword".
This is Kendra Saunders, grandniece of Shiera Hall. Introduced in JSA 1999, and continues through New 52 on Earth 2. This is the Hawkgirl that will be used in the Arrowverse series "Legends of Tomorrow".
It would take me more time to compile a list of comic stories for you to read. I'm studying for exams atm, but I hope this helps for a start. You can probably find issues yourself, now that you know how to distinguish them.