Mycelium wallet is one of the recommended wallet by Bitcoin community. Few ques.
1. Suppose if I put my Bitcoin for 3 years in this wallet and meanwhile they close this app then is there any way to recover it?
On my Android phone Mycelium allows (insists!) to make a backup. The backup consists in a 12-words seed which *should* work on other wallet programs too (e.g. Electrum) to recover your wallet and your money.
I wrote "should" because I use Mycelium only to see the content of my wallet, I don't trust phone security to keep money on a phone wallet, so I didn't try out the backup/restore.
2. What is HD wallet? How it works?
A HD wallet creates all the private keys based on a seed, which is a list of normal words (usually 12 or 24 words) from a list.
This means that if you have the seed you can always retrieve your wallet and all its subsequent addresses.
3. Mycelium is better or paper wallet?
If you want to hold your money for years I advise paper wallet. Just make sure you generate it safely with a known and reputed paper wallet generator.
Obviously you should keep the paper wallet's private key offline (printed? written by hand? just make sure you'll be able to read those letters correctly when you want to spend).
Why I advise paper wallet? Any wallet that stays online is a security risk for your funds. Simple.
