I am using services such as AWS and MS Azure to host websites for my clients. Is it OK / Legal to use those companies logos on my websites as a partner or this is considered infringement?
Amazon says how you may use their logos to describe that your site is powered by AWS.
You can display the “Powered By Amazon Web Services logo” to let your customers know that your application is built on the same reliable, scalable infrastructure used to power Amazon.com’s global web properties.
You will not display the AWS Marks in any manner that implies that you are related to, affiliated with, sponsored or endorsed by us, or in a manner that could reasonably be interpreted to suggest that Your Content, web site, product or service, has been authored or edited by us, or represents our views or opinions.
This seems to say that you can say on your sites that are powered by AWS that they are powered by AWS. It also seems that you can't use them to pass yourself off as a "partner".
They give specific phrasing that you should use:
Example of Permissible Use:
“Application” for EC2
You may replace “for” in the example above with any of the following, so long as the term you use is accurate when used with the AWS Marks you use: “for use with”; “with”; “compatible with”; “works with”; “powered by”; “built on”; “built with”; “developed on”; “developed with.”
You may replace “EC2” in the examples above with any of the AWS Marks, so long as your usage of the AWS Marks is accurate.