When I start Parallels Desktop for Mac on macOS High Sierra I get the following warning:
System Extension Blocked. A program tried to load a new system extension signed by "Parallels International GmbH" and it was blocked. If you want to enable extensions from this developer, go to the Security & Privacy System Preferences pane.
macOS High Sierra 10.13 introduces a new feature that requires user approval before loading newly-installed third-party kernel extensions. Third-party kernel extensions (KEXTs) that were already present when upgrading to macOS High Sierra are granted automatic approval.
For more information please refer to the Apple's official tech note: Technical Note TN2459.
Open System Preferences > Security & Privacy > General > click Allow next to the corresponding message:
Why Parallels Desktop needs its own kernel extensions?
To deliver all Parallels Desktop features, such as virtualization, USB and Network, Parallels Desktop loads the following kernel extensions to macOS:
com.parallels.kext.hypervisor - a proprietary core virtualization driver, which makes Parallels Desktop the best virtualization solution for Mac.
com.parallels.kext.vnic - Shared Network driver, which is used by a virtual router who manages all network services for a VM, such as gateway,DNS, DHCP, port forwarding, network conditioner etc.
com.parallels.kext.usbconnect - USB driver, loaded when a USB device connects to a virtual machine (VM). Supports all USB data transfer protocols except of isochronous transfer.
com.parallels.kext.netbridge - Bridged Network driver, creates a virtual network adapter on Mac to connect a VM to Mac's subnet directly.