Nothing ever changed in the underlying platform as far as I'm concerned, you'd get 100% uptime if there's no issues with the physical host node your vm lies on. If there is, then yes, it could potentially lead to some downtime.
Actually KVM do support live migration, and with a robust network, UPS and diesel generators you'll get as close to 100% uptime as practically possible. Don't think that Google, Amazon or Microsoft actually can guarantee 100% uptime either. They may be closer to 100%, and they may have taken more steps to reduce the risk, but practically it's impossible.