What is the difference between EventBus and RxJava?
EventBus and RxJava are different in their nature.
EventBus is just a bus as the name suggest - it provides the mechanism to subscribe and publish events to the "bus", without you caring how the wiring is done, what this "bus" actually is, etc. In the context of Android, the EventBus is just an easier way to deal with sending and receiving Broadcast messages with less boilerplate.
RxJava on the other hand is much much more powerful than that. Yes, you can subscribe and publish events, but you have far more control over the process - frequency, on which thread everything happens, etc. The main power of RxJava (in my opinion) is that you can manipulate the data being published very easy, using some of its tons of operators.
To sum up - if you only care about publishing some events and performing some actions when received - you'd probably be better off using the simplest of the two, namely some kind of Bus, or even plain old BroadcastReceivers. If you will also benefit of transforming the data, handling threading or simplified error handling - go for the RxJava approach. Just keep in mind that RxJava generally has a steep learning curve, so it takes some time to get used to its concept.
To understand RxJava, think of a list. Today manipulating a list like transforming, splitting, merging can be done easily using functional methods (map, groupBy, etc). RxJava uses the same principles except that its main target is not list but stream. Stream is asynchronous, often live data such as websocket channel or online movie.
Event bus comes from the needs to decouple classes which in Android are often bound with life cycle. Tight coupling of network callback and Activity's Views as an instance, has been a cause of numerous null pointer exceptions. Event bus with its publisher-subscriber pattern alleviates this issue.
How does it get mixed with RxJava ? To begin RxJava incorporates Observable pattern. Here an Observer watches an Observable and reacts when an event arrives. Observable has several sub-classes, among which is Subject that has the properties of both Observable and Observer. Since it works by trapping an event and publishing it to subscribers, it technically functions as event bus.
Is it wise to use RxJava as event bus ? No. RxJava would introduce unnecessary complexities for simpler purposes. Only use it if the app does manipulate streams. For example pairing frames from a movie stream and subtitles from another stream. If the app simply consumes a REST API and needs to decouple the callback from activities/fragments then event bus is enough.