Weight lifting for kyphosis is safe?

Should be: Just be mindful of technique and building up weights slowly over time.

Depends: This depends on what kind of weight lifting you are referring to (technique and amount of weight). Also depends on how big your kyphosis is and why you have the kyphosis. Sorry to leave you with more questions than answers, but no simple answer other than exercise when done properly is almost always good for you.

It can be when: Done in a controlled fashion. If your kyphosis, is any bit severe, you'd be best served by reviewing any resistance training with your doctor(s) first to make sure, it's safe. It turns out the exercise based interventions are actually a reasonable management tool for kyphosis and can at least improved associated spinal muscle strength and physical functioning, and may help with the angulation.