I found origins for Ivanhoe in Hebrew and Slavic. Both were similar in meaning - "a form of John: Gracious gift from God. Ivanhoe is the medieval variant Sir Walter Scott used for the Saxon hero of 'Ivanhoe.'."
there's an investigation of M.Veller, wrighter.
He's found out that Harald Sigurdsson Hardraada, konung of Norway, was married to Elisaveta of Kiev, doughter of Yaroslav I the Wise, the Grand Prince of the Rus .Yaroslav sent to Norway with his doughter a hundred russian warriors as an escort to protect and support her. They were her own personal guards. A man who called himself Ivan Huev was among them. In September 1066, Harald landed in Northern England with a force of around 15,000 men. There were the russians warriors with Harald. Harald died fighting at a final Battle of Stamford Bridge against the forces of King Harold Godwinson of England. Rest of his army left England, but a few of survived warriors joined to William the Conqueror who landed in South England a few days later. Survivors took part in the Battle of Hastings against King Harold Godwinson. After the battle William the Conqueror gave the titles and land to his associates. Ivan Huev became a knight, got some land, married to a saxon girl and stayed in England for the rest of his life. His descendants changed the name by the rules of French grammar. Ivan Huev - Ivanhuev-Ivanhue-Ivanhoe ( reduced ending is a common rule in Old French). Walter Scott found the strange and ancient-sounding name in an old document.
Btw, sobriquet HUEV means in russian "Prick" , "Dick". It looks like Ivan was famous womanizer!
I pretty much figured the IVAN was the form of John, so I did not bother to research that more, this is probably right. I finally ran down the "hoe" part; it is not Celtic, it is old English (Anglo-Saxon) which makes sense, since Scott's character Ivanhoe was Saxon, and it means a bend, or curve, probably in a stream or river - so what we probably have is "John's Bend" - odd that the form "Ivan" is used, though, rather than something more like Johann