Bitcoin's block time is about 10 minutes while that of Ethereum is 15 secs. Both are using PoW at this moment, right? How come Ethereum's block time is so short compared to Bitcoin? Is hashing a lot easier?
Because it was designed to be that way. Ethereum targets 15 seconds, so the difficulty adjusts to make the block interval 15 seconds. Bitcoin targets 10 minutes, so the difficulty adjusts to make the block interval 10 minutes. It is unrelated to how difficult it is to produce a hash; it is only related to what hashes are considered valid.
The set of hashes considered valid is set by the difficulty adjustment. Making it smaller means that it will be take longer or require more hash rate to find a valid block hash. Making it larger has the reverse effect. So Ethereum has a larger set of valid hashes than Bitcoin, so valid block hashes can be found more quickly as fewer hashes need to be tried, on average.