My shower valve has cold water when the upstairs kitchen tap is on

Do you have heat trap nipples installed on your water heater?

Do you get hot water everywhere else in the home even with the tap open? How well does the shower operate with the tap closed? Sounds like you have a volume problem likely due to a restriction

Hold on here...

You say, "my shower valve has cold water when the upstairs kitchen tap is on", and no hot water at all? Please explain this more...not clear??

Do you have cold water even without the kitchen tap open??

Back to you...

Mark