My son, who's studying history, asked me about the word 'shew' which he often comes across. 'Shew' is an old variant of the verb 'show'. It was pronounced like 'shoe'. It is found a lot in the Bible, and the OED says it was not uncommon in the first half of the 19th century but is obsolete now. Chambers doesn't go as far as to say it is obsolete; at the 'show' entry it has 'shew' which has before it the words 'now rarely'. The internet is full of comments on forums from people who insist that they, or their relatives, were using the 'shew' spelling in the 1940s, and the respected late journalist Miles Kington insisted in this article that he had seen a sign at a railway station in the 1940s that read 'Tickets must be shewn at entrance to platform'.
'Show' is an interesting verb because it has a mixture of strong (vowel-change) endings, and weak ones. The past tense is 'showed' (I showed him my etchings) and the past participle is usually 'shown' (Have you shown her your etchings?), on the same pattern as blow - blown and know - known. The past tense of 'shew' is, or was, 'shewed', and the past participle was 'shewn'.
I said the past participle of show is usually shown, but you often see 'have showed'; there are several examples on today's Google News pages, and that's probably fairly typical (eg 'new figures have showed' in this Press Association article). You are less likely to see showed as the past participle in a passive sentence, however; so 'it was shown' is much more common than 'it was showed'.