Senior Member. "showed" is the simple past, "shown" is the past participle.

Were shown or are shown?

If you're using the passive voice, along with “is” or “was” or “are” or “were,” then “shown” is preferred, as in “he was shown the door” or “the paintings were shown to be forgeries” or “names are shown in italics.”

Is showed the past tense of show?

past tense of show is showed.