does the XL-H1 (or any other DV/HDV) have a luminance or RGB histogram display like so many DSLRs do?

Unfortuntely no, none of them do. That would have been a very cool feature though.

...why Canon XL H1 don't have it, it's more then a cool feature - really helpful! And I think it's strange - because I have it in my Sony HDR-HC1E which is a prosumer cam.

do most pro cameras have this feature - sony dsr 300/500 series?

I'd really like to have a histogram display too but if you think about it the most common thing we do with a histogram is look to see if anything is sloshing up the right hand wall of the thing. The zebra stripes give that information to you and in addition tell you not only that you are saturating but where.

... and, if I remember right, the Console program includes such a tool.