an invocation is the action of invoking something or someone for assistance or as an authority and in this poem as you can obviously tell that's what their doing.
I would call this an ode which is a lyric (musical) poem addressed to a particular subject and sometimes it can be sung. I also think of an ode as showing reverence for someone as this poem obviously does as the speaker talks of worshippiing the object of his/her affections.
