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I have a Canadian visa (on a passport that's expired) valid until May 2019. I had to get a new passport issued due to lack of pages. Now if I have to travel to Canada this year, can I get a visa renewal on my new passport, as I wish to use it with my new passport, while it's still valid.

Thanks.

You can apply for a new visa, but you do not need to. You can enter Canada using your new passport along with the valid visa in your old passport. See Can I travel to Canada with valid visa on old/expired passport? for more information.

Note also that the visa need not be valid for your entire stay. For example, you could use your existing visa to enter Canada in April 2019 for three months, even though it expires a month after you would enter. See Temporary resident visa validity (expiry dates):

The expiry date on the TRV is the date by which the visa must be used to seek admission into Canada. If the visa is not used on or before that date, it ceases to be valid.

The validity date is NOT the suggested duration of the visit. At the port of entry examination, CBSA officers determine the length of time temporary residents are authorized to stay in Canada.

Typically, when officers are satisfied that the foreign nationals are admissible, entry is granted for a six-month period, unless they hold a Parent and Grandparent Extended Stay Temporary Resident Visa (PG-1 or letter of introduction from the visa office), in which case entry may be granted for a period of up to two years on their initial stay.