It seems that there's a lot of confusion about these terms of late.

Values. These are the ideals that a group of people embrace. They can be positive or negative - e.g. empowerment or control. These values are implicit in the personality or culture of a company. Values are often emotive - they represent driving forces behind people.

Principles. Although the word stems from the Latin for leader or emperor, in this context we're talking about it as a general law or essence - e.g. principles of modern physics

Practices. The easiest one to grasp - a set of repeatable actions you perform. Practice the flute. Practice loyalty. Practice developing software by driving with tests. OK, that one's easy.

So how do these terms tie together? A practice works in a given context due to an underlying principle or principles. For example, the practice of employing small easy-to-adjust tooling machinery in place of one large cumbersome automated device is more productive for a manufacturing line with varying output requirements. The underlying principle is basically to 'smooth the value stream' - reducing bottlenecks to enable flow. Continuous integration is a software development practice that is backed by the same principle. For an individual to adopt a practice they must see that the practice works in the field and understand the principle(s) behind it.

How are values related? Practices produce effects that support one or more values.If a development organisation values the ability to meet their customers needs (rather than make the customer fit their needs as so often happens), then a practice such as TDD will support that value as it keeps the cost of change low over time (the principles, to name but a few: once and once only, simplest thing that works). This is where Agile/XP teams often run into difficulty with organisational structure - many of the practices support values that are not necessarily consistent with those of the organisation (empowerment being a key example). Worse still, a company may have no common values (perhaps due to a lack of common vision or leadership) or different elements of a company may hold conflicting values.