From Teflpedia

Show /ˈʃəʊ/ is an English verb.

Show is usually an irregular lexical verb - it has the third person form "shows" /ˈʃəʊz/, the -ing form "showing", usually a regular preterite "showed" but usually an irregular past participle "shown". However, "showed" is also sometimes used as a past participle, making "show" regular. Garner's Modern American Usage favours shown over showed as past participle and claims it is mandatory for passives.