A Sestina is a very complex verse form, usually unrhymed, consisting of six stanzas of six lines each and a three-line envoy. The end words of the first stanza are repeated in a different order as end words in each of the subsequent five stanzas; the closing envoy contains all six words, two per line, placed in the middle and at the end of the three lines.
But you do not really make the quite hard analysis of the poem because the poem itself is named “Sestina” and it was written by Algernon Charles Swinburne
So, to give a final answer to this question, this excerpt is an example of:
C.Sestina