I've been under the impression that a historical drama is non-fiction whereas a period drama is fiction.
"Drama" is usually meant to indicate fiction.
I've generally understood historical/period drama as:
- Historical drama - a drama based on historical events
- Period drama - a drama based in a particular time or period, not obviously based on real events
So, The Crown would be a historical drama
Follows the political rivalries and romance of Queen Elizabeth II's reign and the events that shaped the second half of the 20th century.
and Downton Abbey would be a period drama
A chronicle of the lives of the British aristocratic Crawley family and their servants in the early 20th Century.
(the Crawleys being a fictional family where Queen Elizabeth II obviously isn't)
However, the term is somewhat ambiguous and commentators may use them interchangeably, so don't expect my opinion to be rigidly observed by all.
I'd expect a historical non-fiction programme to be called a "historical documentary" rather than a "drama", even though the event(s) portrayed might be dramatic in nature.