Although Hong Kong is officially part of China, it is politically and economically a separate entity and Hong Kong continues to use the Hong Kong dollar as its official currency. Almost all shops, restaurants and other business will only accept the Hong Kong dollar as payment. The Hong Kong dollar is widely available both in Europe and the US.
That said, China's increasing influence over the city and an influx of Chinese mainland businessman in recent years has seen a number of large shops start accepting Yuan. Wellcome and Park n Shop supermarket are just two examples. Shops that accept Yuan will display a sign at their register, however, you should be aware that the exchange rate you receive at these stores will not be good.
Those planning to travel via Hung Hom to China will be able to make payment in Chinese Yuan and some travel agents, including the ubiquitous CTS will also accept payment in Yuan.