JAVA - Java is a general-purpose computer-programming language that is concurrent, class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible.
AJAX - You can create a complete website knowing just this, however, it'll look like those 90s websites with blue links. It's there to create a basic structure for a website and it's impossible (as far as I know) to make any website/webpage without using HTML.
XML - It's not related to web pages at all. It's just a way of storing data wrapped in XML files. The text data wrapping is used so that we don't have to use databases. You wouldn't want to send a complete database table when you just need the current weather information, would you? It's not just used to send data, sometimes it's used to represent User-interfaces too (in Android and iOS, while Windows uses XAML for that). The pages in your Android app are generally coded in XML, which the Android system (technically Dalvik VM or nowadays ART) reads and convert it to proper Java. So it's just for storing textual info which gets parsed and the parsing/reading mechanism uses it for their own purpose. The syntax looks like HTML tags.
To read more about XML try the following answer