How to connect ESP8266-01 (AT-commands) to its web server using AJAX
You're trying to mix code that is intended to run directly on an ESP8266 using ESP8266-specific libraries on an Arduino.  It can't ever work.

Likes like:

server.on("/",WebsiteContent);
server.on("/xml",XMLcontent);
server.begin();


are for use with the ESP8266's ESP8266WebServer library which doesn't run on an Arduino. To use these kind of constructs you have to program the ESP8266, not the Arduino, and then you don't have the AT commands available. 

So, either:


Do it all with AT commands only, or
Ditch the Arduino and program the ESP8266 directly instead.