If you're developing apps for all the major mobile platforms i guess you are using a Mac since that is the only way you can build iOS apps. If you're using Parallels to run Windows for Windows Phone development you need to activate nested virtualization if you want to use the emulator. This is because that Windows Phone emulator is using virtualization with Hyper-V.
To activate nested virtualization, shut down your virtual machine if it is started. Then open the configuration view for your machine in parallels and go to the "Optimization tab" and check "Enable nested virtualization". When you start your virtual machine you'll be able to use Hyper-V and the Windows Phone emulator.