I'd like you to help me to choose the right technology that fits my requirements with least costs.
The android app needs to constantly send the data back to server (The data like heartbearts), as well as constantly receive the feedback data from server (Real-time data, streaming)
The data type can be number, text, or even graph.
I'm now looking at three technologies
I'm not familiar with any of that, please give your suggestions, thank you.
Realtime bi-direction full-duplex communication = WebSockets.
IMHO WebSockets will become the standard for server <-> client communication. It's still reasonably early in their development, in terms of non-browser solutions, but the amount of options available to developers is increasing all the time. jWebSockets look like a good solution for Android and there are a host of WebSocket servers that are available to choose from. I work for Pusher who offer a hosted WebSocket service and there are other similar services that you can investigate.
Have a look at this realtime technologies guide for a list of technologies (I'm trying to keep this up to date as much as possible as I find new technologies).