Google Compute Engine has live migration, so host/hypervisor updates (including security updates!) can be applied without taking VMs offline. AWS doesn't support live migration.
I don't work for AWS, so I can only speculate about their internal technology; but they've demonstrated an ability to apply security updates without requiring guests to reboot.
I don't work for AWS, so I can only speculate about their internal technology; but they've demonstrated an ability to apply security updates without requiring guests to reboot.