(No major change in WS2019)Server 2012 / R2 Hyper-V allows to hot-add memory to the virtual machine as long as the memory is configured as dynamic, essentially allowing to adjust the memory while the vm is up and running. Server 2016 allowed to hot-add static memory and network adapters to the running VMs.
Since 2008R2 Hyper-V allows to add virtual storage (only when connected to virtual SCSI adapter - not IDE) is supported while the vm is running.
Windows Server 2016 enables to hot add/remove virtual NIC and static memories. Windows Server 2016 doesnt support the hot add vCPU. However guest applications as SQL Server need to be restarted when a vCPU is added to the VM; even on VMware