I am trying to install OS X on VirtualBox VM on Windows 7, but it is giving me a message that "hardware acceleration" is missing on my PC.
How can I deal with this problem? …and is this suitable for iOS app development using Xcode?
The OS X license does only allow to install a virtual copy on Apple hardware.
Due to Apple licensing restrictions, you may only create and run this virtual machine on Apple-labeled hardware. For more information, see Apple's Hardware & Software Product Agreements.
You recieve this error probably because your CPU doesn't support Hardware Virtualization (VT-x for Intel CPUs). Changing CPU would probably resolve the problem, but it cannot be suitable to do good iOS developing due to bad overall performance. And, as Matteo said, do things like these (i.e. hackintosh, OSX VM without a Mac) is against Apple terms and conditions.