Kernow is the Cornish language name of Cornwall to this day, with cognates in Welsh Cernyw and Breton Kernev. (Kernev is also the Breton form of the region of Brittany known in French as Cornouaille.) Its Latin name is Cornubia, but it was known to the Anglo-Saxons of neighbouring Wessex as the kingdom of the West Welsh, later as Cornwall.
Two waves of migrations took place to Armorica (Brittany) from Dumnonia and Cornwall and this may have resulted in rulers who exercised Kingship in both Brittany amnd Cornwall, explaining those occurences of the same rulers names in both places.
Cornwall had remained largely un-Romanized and settlements continued in use into the post-Roman period. It is suggested that the kings were itinerant, stopping at various palaces, such as Tintagel, at different times of the year. Lesser lords built defended 'rounds' like Kelly Rounds and Castle Dore.
Cornwall may have reverted to paganism after the Roman departure from Britain, or perhaps Christianity never reached these far-flung parts of the Empire. In the 5th and 6th centuries, however, the area was evangelized by the children of Brychan Brycheiniog and saints from Ireland. There was an important monastery at Bodmin and sporadically, Cornish bishops are named in various records until they submitted to the See of Canterbury in the mid-9th century.
In the De Gestis Herwardi Saxonis written in the 12th century it is recorded that Hereward the Wake took refuge in Cornwall in the 11th century at the court of the Cornish Prince or King Alef.
Since the 19th century, there has been controversy concerning a certain Huwal, "King of the West Welsh". This character only appears in the Anglo-Saxon Chronicle entry for 927, accepting King Athelstan of Wessex as his overlord. 'West Wales' was an old term for Dumnonia or Cornwall, but may also refer to present day West Wales, then generally known as Deheubarth, where Hywel Dda was king. Other 'kings', such as Ricatus, mentioned on memorial stones may have ruled more localised regions.
An early 17th century pedigree of a so-called 'Earl of Cornwall' in the Book of Baglan may possibly also represent a list of rulers in Cornwall
According to William of Worcester, writing in the 15th century, Cadoc, described as the last survivor of the Cornish royal line at the time of the Norman Conquest in 1066, was appointed Earl of Cornwall by William I of England.
Lying in the extreme west of Britain, Cornwall was protected from Anglo-Saxon land invasions until 814 when King Egbert of Wessex subdued parts of Devon that were until then part of Cornwall. Clashes continued throughout the early 9th century and by the 880s Wessex had gained control of at least part of Cornwall, where Alfred the Great had estates. William of Malmesbury, writing around 1120, says that King Athelstan of England (924–939) fixed Cornwall's eastern boundary at the Tamar. The chronology of English expansion into Cornwall is unclear, but it had been absorbed into England by the reign of Edward the Confessor (1042–1066). Cornwall showed a very different type of settlement pattern to that of Saxon Wessex and places continued (even after 1066) to be named in the Celtic Cornish tradition with Saxon architecture being uncommon in Cornwall. The earliest record for any Anglo Saxon place names west of the Tamar is around 1040.