Transparency
(the definition by book)
Significant aspects of the process must be visible to those responsible for the outcome. Transparency requires those aspects be defined by a common standard so observers share a common understanding of what is being seen.
Openness
(the definition by book)
As we work together, we express how we're doing, what's in our way, and our concerns so they can be addressed.
What's the difference and my interpretation
Transparency refers more on having a common language, that everybody inside and outside the team understand. Which is achieved by having a transparent access on knowledge and information.
Openness refers not only on having transparency but also on having a collaborative and/or cooperative managment and decision-making.
For example sharing Goals, DoD, DoR, User Stories, Burn-down chart brings transparency up.
Openness is about to say what have need to be said. Without blaming but with open mind and courage. For example when we describe an impediment or we reject a unready User story or propose a new breaking idea.
It's not about saying everything to everybody but the right things that could probably hurt a bit but are needed in order to reach goal and preserve values.
That's my opinion, what's yours?
