A paradox is a contradiction; a complication or conflict.
So, if we take a look at the last verse of this poem
That when we live no more, we may live ever
We find that there is a direct conflict in the concept of life, since life and death are juxtaposed in a way that contradicts the message.
When we live no more= Death
We will live ever= We will live forever
So, we will live forever after dying?
To a poet, or to someone who is quite religious this concept may seem plausible, since the idea behind most systems of faith is that there is an afterlife for those who are good and faithful.
However, someone reading the words literally will find that there is no way to "live ever" after death, because death is final.
Granted, the entire poem is about fostering love in a way that it becomes strong enough to resonate through time and space, kind of like Romeo and Juliet, whose romance continues to be iconic through the centuries. So, in a way, this is a way to be "immortal" and live forever.