如果剛移民加拿大,還可以保持香港永久居民權嗎?

yes, you still own the HK resident right , even you become Canada citizen.
any time you can come back to live in HK and work. you are always HK people until year 2047.

香港永久居民權 is difference with 國藉.
香港永久居民權 (Resident) - means u can stay/live/work in HK. u can be other country citizen, depending which country passport u have. whatever u are holding US passport/Canada passport, u still can keep the 永久居民權/HKID card and use it.
國藉 (Citizenship) - means u are the citizen and have its passport. China government does not allow Chinese ppls have dual-citizenship (have another citizenship beside China); so once u get other country citizenship, u cannot use the HKSAR passport anymore (Chinese passport), but u still can keep ur HKID card.

自回歸後,香港是否和中國一樣只保留一國藉?

可以