What is the "main" difference between consensus mechanism of Bitcoin and Ethereum? Bitcoin and Ethereum both use proof-of-work (PoW) consensus mechanism. One of differences between Bitcoin Pow and Ethereum PoW is difficulty level, meaning that while in bitcoin block generation rate is 10 minutes on average, in Ethereum is around 12 seconds on average.

Is there any other major difference between these two PoW algorithm? 
For example, does Ethereum use proof-of-stake (PoS) in parallel?
The base proof of work system only differs on the block time, and the hashing algorithm. Bitcoin uses sha256d, ethereum uses ethash.

Ethereum currently does not make use of Proof of Stake, although it is planned for the future. Some of the testnets use Proof of Authority.

The consensus rules differ by a fair extent, however, since the two chains follow different models. Bitcoin is a utxo based system, and the consensus rules revolve around that. Ethereum is an account based system, and has its own consensus rules to deal with that.

Moreover, ethereum has a much more complete smart contract platform, which requires additional rules, such as ensuring transactions are executed in order.