I contacted the Guernsey Information Centre, North Plantation, St Peter Port, GY1 2LQ
There response was simply:
"Sorry we do not have hostels in Guernsey only normal camp sites"
There is a fairly comprehensive listing of Guernsey accommodation on Visit Guernsey
Unfortunately there are indeed no hostels listed. There are a variety of guesthouses, inns and self-catering options.
Or another option might be Couchsurfing, since there are a few hosts there.