If I use a feat, let's say Taunt, do I get the AV +1 only when using the feat (seems right) or do the character always gets +1 to hit?

Only when using the feat. Some feats, like Swingline, are basically always active, so you'd constantly have a +1 to movement. Others, like Pounce, Haymaker, Large Object, etc. only work when you want them to.