Measuring voltage while maintaining isolation
You're looking for an isolation amplifier.  These are great if you're looking for a monolithic solution that is easy to integrate, but might not be cost-optimized. Some even provide a little isolated (HV-side) power via an internal DC/DC converter to run scaling op-amps, etc.

I've had good luck with the Burr Brown ISO series of isolation amplifiers (acquired by TI a few years back). The ISO122 is a general use product, and the ISO124 has higher accuracy.

Analog Devices also makes a few general purpose isolation amps.  

Just be aware that some sort of modulation is required to move the signal across the isolation barrier within an isolation amplifier.  Some do it inductively, some optically.  Either way, plan on a little ripple on the output signal.  The specs usually do a good job of outlining the limitations.