How to counter Xayah
Let us take a look on how to beat Xayah. We will look at champions who are strong against Xayah and who are weak against Xayah. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Adc champion who counters Xayah. You can already win in champion select with the right counters for Xayah. If you want to know how to counter Xayah then you need to take into account the best picks vs Xayah and the worst picks vs Xayah. This Xayah counter data is based on matches played by Pros on patch 15.11 and previous ones. It is important to see whether Pros counter picked Xayah or whether they played as Xayah.
Champions used by Pros to counter pick Xayah
Looking at the matches from Pros (see table Pros counter picked Xayah) we can see that they picked 
Kai'Sa most often to counter pick Xayah. The matchup Kai'Sa vs Xayah occured 77 times. Xayah has 37 wins and 40 losses in this matchup. Learn how to counter pick Kai'Sa to get another perspective on the data.
Lane outcome for Xayah
When looking at Xayah counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Xayah. The lane outcome of Xayah consists of three outcomes: Win, Loss or Draw. Let's take a look of Xayah lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Xayah.
Conclusion
Analyze the available data and learn how to beat Xayah. To master Xayah take also a look on Xayah builds and runes for Xayah. Good luck, Summoner!