In modern Chinese for daily talk, we use "狗" to refer to "dog",  while “犬” is usually used in words like "猎犬" “恶犬” "犬齿" or some idioms like "画虎不成反类犬", in short, you can use "狗" for most cases. “犬” is more like a zoology term for “狗”

我养了一条狗

这条狗是猎犬（hound）

“犬” is more formal and traditional at the same time. So you can see it used in many idioms. Because most of idioms were developed from traditional Chinese articles.

“狗” is less formal and very common in our daily conversation.  

狗的量词有“只、条”

我有两条狗。其中一条比另一条大一些。