GENUKI's gazetteer entry for Kenwyn says:
The parish of Kenwyn, (Cornish: Keynwynn), is situated in the Deanery and Hundred of Powder. It is bounded on the north by St Allen, on the east by St Clements, the parish of St Marys Truro and the river Fal, on the south by Kea, and on the west by St Agnes, the detached part of Kea called Tregavethen, and Perranzabuloe.
From British History Online's digital version of Lewis' Topographical Dictionary:
TREGAVETHAN, a manor and extra-parochial district, in the parish of Kea, union of Truro, W. division of the hundred of Powder and of the county of Cornwall; containing 52 inhabitants. The manor comprises 1024 acres, of which 389 are common or waste land. It is situated on the borders of Kenwyn parish, and an aisle in that church is appropriated for its population. The impropriate tithes have been commuted for £70. 16. 8., and the vicarial for £22. 16. 8.
And from Wilson's Imperial Gazetteer at A Vision of Britain, the entry on Kenwyn says, in part:
The sub-district contains also the extra-parochial tract of Tregavethan.
Searching for Trehavron and for Tregavran/Trehaverne yielded no results.