If the german company has a VAT number, then no vat is charged by the hk company. But the german company has to pay it (german vat rate) to the german gouverment (reverse-charge mechanism).
If the german company did not have a vat identification number, hk should charge german vat on the invoice.
If you receive services for business purposes from a supplier based outside the EU, you should usually pay VAT at the applicable rate in your country, as if you had supplied the service yourself (using the reverse charge procedure).